Development of LTE Network

8/7/2019 Development of LTE Network

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MANIPAL INSTITUTE OF TECHNOLOGY(A constituent Institute of MANIPAL UNIVERSITY)

MANIPAL - 576 104, KARNATAKA, INDIA

Mid-Semester Report

Development of LTE Network

SUBMITTED

BY

ANKUR KHANNAELECTRONICS AND COMMUNICATION

070907496

UNDER THE GUIDANCE OF: Dr. Brejesh Lall

Associate ProfessorElectrical Department

IIT Delhi



CERTIFICATE OF ACK NOWLEDGEMENT

10th

March 2011

To whomsoever it may concern,

I acknowledge that Ankur K hanna has been working on a project under my

guidance on the µDevelopment of LTE network¶ at IIT Delhi. I understand that thefollowing is a report of the work he has done so far and an estimate of what he will

do in the coming weeks. The following study has been done individually by him

and this report is an accurate account of it.

Dr. Brejesh Lall

Associate Professor

Electrical Department

IIT Delhi



Contents

1 Acknowledgement

2 Introduction

2.1 The LTE Standard

2.2 Need for the Project

2.3 Objective

3 An Introduction to LTE

3.1 Network Architecture

3.2 Layer 2 Structure

3.3 Downlink Channels

3.4 Physical Layer Protocols

4 PHY Layer Architecture

4.1 Encoder

4.2 Spatial Multiplexing

4.3 Transmit Diversity

4.4 OFDM

4.5 RF

4.6 Miscellaneous

5 Overview

6 References



Acknowledgement

This project has been an individual effort of mine. However, it would not have been possible

without the kind support and help of many individuals and organizations. I would like to extendmy sincere thanks to all of them.

I am highly indebted to Dr. Brejesh Lall, Associate Professor, Electrical Department, IIT Delhi, for

his guidance and constant supervision as well as for providing necessary information regarding

the project.

I would like to express my special gratitude and thanks to my guide Mr. Shashi Kumar G. S. for

giving me such attention and time.

I would like to express my gratitude towards my parents for their kind co-operation andencouragement which helped me to work diligently on the project.

My thanks and appreciations also go to my college in developing the project and people who

have willingly helped me out with their abilities.



Introduction

2.1 The LTE Standard3GPP Long Term Evolution (LTE) is the latest standard in the mobile network technology tree that

produced the GSM/EDGE and UMTS/HSPA network technologies. It is a project of the 3rdGeneration Partnership Project ( 3GPP), operating under a name trademarked by one of theassociations within the partnership, the European Telecommunications Standards Institute.

The LTE specification provides downlink peak rates of at least 100 Mbps, an uplink of at least 50Mbps and RAN round-trip times of less than 10 ms. LTE supports scalable carrier bandwidths,from 1.4 MHz to 20 MHz and supports both frequency division duplexing (FDD) and time divisionduplexing (TDD).

The main advantages with LTE are high throughput, low latency, plug and play, FDD and TDD inthe same platform, an improved end-user experience and a simple architecture resulting in low

operating costs. The next step for LTE evolution is LTE Advanced, a 4G Technology, and iscurrently being standardized in 3GPP Release 10.

2.2 Need for the ProjectModern day advancements have created an environment where the flow of information has becomea necessity not just for a specific field but for all facets of the civilization. The need for betterperformance and reliability has resulted in the development of better modes of communication andthe process has not stopped.

Higher data rates demand the implementation of better techniques and standards in the hierarchy of wireless communication. Currently the most popular standard is the 3rd Generation of MobileCommunication. LTE is the advancement from 3G to 4G and will introduce a new era of highspeed data transfer.

2.3 ObjectiveThe idea of my project is to design an LTE network that meets the 3GPP specifications for the LTEStandard. In particular the PHYSICAL Layer of the devices (the Base Station or the eNode-B or theUser Equipment) has to be studied and designed with the necessary channel coding and modulation

techniques required as per the LTE Standard. Other important aspects of this layer are transmitdiversity and MIMO configuration for transmitters and receivers. Moreover, collaboration withhigher layers of the OSI model or the IP model as followed by the devices in the network is desired.

This requires an initial study of the standards proposed by 3GPP and IMT and then developing adesign for the basic elements of the network. The project is considered from the point of view of the Base Station.



An Introduction to LTE

3.1 Network Architecture

All the network interfaces are based on IP protocols. The eNBs are interconnected by means of anX2 interface and to the MME/GW entity by means of an S1 interface as shown in Figure 3.1. TheS1 interface supports a many-to-many relationship between MME/GW and eNBs.

Figure 3.1: LTE Network Architecture

The P-GW interfaces with external packet data networks (PDNs) such as the Internet and the IMS.The P-GW also performs several IP functions such as address allocation, policy enforcement, packetfiltering and routing.

The MME is a signaling only entity and hence user IP packets do not go through MME. Anadvantage of a separate network entity for signaling is that the network capacity for signaling andtraffic can grow independently. The main functions of MME are idle-mode UE reachability

including the control and execution of paging retransmission, tracking area list management,roaming, authentication, authorization, P-GW/S-GW selection, bearer management including dedicated bearer establishment, security negotiations and NAS signaling, etc.

Evolved Node-B implements Node-B functions as well as protocols traditionally implemented inRNC. The main functions of eNB are header compression, ciphering and reliable delivery of packets. On the control side, eNB incorporates functions such as admission control and radioresource management. Some of the benefits of a single node in the access network are reducedlatency and the distribution of RNC processing load into multiple eNBs.



3.2 Layer 2 Structure

The layer 2 of LTE consists of three sublayers namely Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Convergence Protocol (PDCP). The Service Access Point (SAP)between the physical (PHY) layer and the MAC sublayer provides the transport channels while theSAP between the MAC and RLC sublayers provides the logical channels. The MAC sublayerperforms multiplexing of logical channels on to the transport channels.

The downlink and uplink layer 2 structures are given in Figures 3.2. The difference betweendownlink and uplink structures is that in the downlink, the MAC sublayer also handles the priority among UEs in addition to priority handling among the logical channels of a single UE. The otherfunctions performed by the MAC sublayers in both downlink and uplink include mapping betweenthe logical and the transport channels, multiplexing of RLC packet data units (PDU), padding,transport format selection and hybrid ARQ (HARQ).

Figure 3.2: LTE Layer 2 Structure

The main services and functions of the RLC sublayers include segmentation, ARQ in-sequencedelivery and duplicate detection, etc. The in-sequence delivery of upper layer PDUs is notguaranteed at handover. The reliability of RLC can be configured to either acknowledge mode (AM)or un-acknowledge mode (UM) transfers. The UM mode can be used for radio bearers that cantolerate some loss. In AM mode, ARQ functionality of RLC retransmits transport blocks that failrecovery by HARQ. The recovery at HARQ may fail due to hybrid ARQ NACK to ACK error orbecause the maximum number of retransmission attempts is reached. In this case, the relevant



transmitting ARQ entities are notified and potential retransmissions and re-segmentation can beinitiated.

The PDCP layer performs functions such as header compression and decompression, ciphering andin-sequence delivery and duplicate detection at handover for RLCAM, etc. The header compression

and decompression is performed using the robust header compression (ROHC) protocol.

3.3 Downlink Channels

The relationship between downlink logical, transport and physical channels is shown in Figure 3.3. Alogical channel is defined by the type of information it carries. The logical channels are furtherdivided into control channels and traffic channels. The control channels carry control-planeinformation, while traffic channels carry user-plane information.

Figure 3.3: Downlink Channels

In the downlink, five control channels and two traffic channels are defined. The downlink controlchannel used for paging information transfer is referred to as the Paging Control Channel (PCCH).This channel is used when the network has no knowledge about the location cell of the UE. Thechannel that carries system control information is referred to as the Broadcast Control Channel(BCCH). Two channels namely the Common Control Channel (CCCH) and the Dedicated ControlChannel (DCCH) can carry information between the network and the UE. The CCCH is used forUEs that have no RRC connection while DCCH is used for UEs that have an RRC connection. Thecontrol channel used for the transmission of MBMS control information is referred to as theMulticast Control Channel (MCCH). The MCCH is used by only those UEs receiving MBMS.



3.4 PHY Layer Protocols

The protocols involved in the development of the Physical Layer for an LTE standard network fordownlink transmission are:

y OFDMA is the modulation scheme defined to meet the specifications.

y Channel Coding is done using Turbo Coding, Convolution Coding as well as Block Coding.y LTE Standard supports MIMO spatial multiplexing.

y The transmit diversity to support and implement MIMO transmission is basically SFBC-FSTD(Space Frequency Block Code ² Frequency Shift Transmit Diversity) along with CyclicDelay Diversity(CDD) when required.

y Frame Structures follow both FDD (Frequency Division Duplex) and TDD (Time DivisionDuplex).

y Synchronization signals are generated using ZadoffîChu (ZC) sequences.

y Hybrid ARQ with Incremental Redundancy (and Chase Combining if required) is employed.

y Precoding is included in Transmit Diversity for increased spectral efficiency.

y Dedicated channel for Multicast signals.



PHY Layer Architecture

Figure 4.1: PHY Layer Architecture

The Physical Layer Architecture that I am designing consists of various modules that representfunctions or libraries that will be implemented either physically of logically.The Architecture can be basically divided into a 5 modules apart from the CPU as shown in Figure4.1.These modules can be further broken down into smaller functions that are explained later.

The architecture is designed keeping in mind that data will primarily be identified by the TransportChannel that it is associated with. These transport channels are defined in the MAC Layer and thedata from different transport channels will be processed according to the corresponding physical

channel that is connected to each transport channel.

4.1 Encoder

This module directly receives data from the MAC Layer and the basic function of this module is toencode the data. In other words it is responsible for channel coding. A basic block diagram of thismodule is shown in Figure 4.2.



Figure 4.2: Encoder

The internal blocks of this module will be described now.

4.1.1 Code Generator

This is basically a code generator that generates different codes that are necessary to transfer ascontrol signals. These codes are as follows:

y DCI- Downlink Control Information is a set of bits that is transferred over the PhysicalDownlink control Channel.

y CFI- Control Format Indicator indicates the number of OFDM symbols (i.e. normally 1, 2or 3) used for transmission of control channel information in each subframe. It istransmitted over the Physical Control Format Indicator Channel.

y Synchronization Signals- The Primary Synchronization Signal is generated using Zadoff-Chusequences and the Secondary Synchronization Signal is based on the maximal lengthsequences. These signals are transmitted over the Physical Broadcast Channel.

y Reference Signals- These are generated using PN sequences and are use for variousdownlink measurements.

y HARQ Indicator- This is transmitted over the Physical HARQ Channel and includesinformation regarding the ARQ response.



4.1.2 CR C Attachment

This block is calculates the CRC (Cyclic Redundancy Check) of a block of bits as a method for errorcorrection in compliance with the Hybrid ARQ scheme.The Transport Blocks that are passed from the MAC Layer contain a large number of bits and so

they are broken down into Codeblocks and CRC is attached to each of these Codeblocks for betterefficiency. This is illustrated by Figure 4.3.Out of the 3 schemes, Scheme C is chosen to attach CRC to the TBs and CBs.CRC calculation can either be done logically or it can be implemented using Linear Feedback ShiftRegisters. A primitive (irreducible) polynomial is used based on the size of the CRC that is to beadded and the incoming set of bits is divided by this polynomial. The remainder is attached as theCRC to the input set of bits.

Figure 4.3: CRC Attachment for efficiency

The different schemes are shown in Figure 4.4.

Figure 4.4: CRC Attachment Schemes



4.1.3 Code Block Segmentation

As mentioned earlier, the Transport Blocks are broken down into smaller units called Code Blocks.The sizes of these Codeblocks are predefined for convenience and if the necessary padding bits areadded to meet the requirements. Even here there are a few issues concerning the optimum sizes of

the codeblocks. If the sizes of the codeblocks vary drastically then after encoding there will be acorrespondingly huge difference in the rates. This is why an algorithm is followed to keep the sizesof the codeblocks close to each other. Figure 4.5 illustrates the difference in the sizes.

Figure 4.5: CodeBlock Segmentation

Even though the number of filler bits being added in the second case is more than that in first, yetthe second case is a better option because the sizes of the codeblocks in the second case are muchcloser.

4.1.4 Channel Coding

To meet the specifications laid down for the LTE standard, channel coding of data is done either by Turbo Coding, Convolutional Coding or Block Coding. Figure 4.6 shows the channels and theircorresponding coding schemes.



Figure 4.6: Channels and their coding schemes

Turbo Coding The bits that have to be encoded are given as input and for each input bit there are 3 output bits.That is, there are 3 streams on the output side. The first stream is comprises of the same bits thatwere sent as the input. This stream is called systematic bits. The other two streams (called Parity 1and 2 bits) are the output of the encoders. Refer to Figure 4.7. There are a few issues concerning thisscheme such as filler bits removal and trellis termination.

Another important aspect of the turbo coder is the interleaver used. For the sake of simplicity I willnot include the logic for the interleaver(which involves the use of Quadratic PermutationPolynomials), but it will be considered during implementation.

Figure 4.7: Turbo Coder



Convolutional Coding Similar to Turbo Coding in the way that the rate is 1/3 i.e. for each input bit there are 3 output bits.

Figure 4.8: Convolutional Coding

4.1.5 Scrambler

It consists of interleaver and interlacer logic. Apart from that, this module is also used as interleaver

in another block that we will see later. The function of the interlacer will become clear from Figure4.9.

Figure 4.9: Coding



4.1.6 Rate Matching

It is not necessary that the buffer size of the UE receiver would be same as that of the transmitter. Itis generally smaller. The soft buffer size is directly proportional to the supported data rate and isinversely proportional to the turbo coding rate. The idea with soft buffer limitation is that if UE has

a certain buffer size dimensioned for a given data rate and a given coding rate then it can supporteither higher data rates with increasing coding rate (weaker code) or lower data rates with a strongercode. Therefore, UE can support incremental redundancy in most cases while falling back to Chasecombining when it approaches peak data rates if its soft buffer is dimensioned to support Chasecombining only at the peak data rate. In other cases, when the soft buffer is dimensioned to supportthe peak data rate with incremental redundancy in mind, full redundant bits can always betransmitted providing performance advantage. Therefore, the soft buffer approach provides a toolto balance the UE soft buffering complexity against incremental redundancy gains.

This is done by using a buffer which is logically a circular buffer and resets to position ¶0· after itsmaximum value which is decided by the limiting value of the soft buffer.

To implement incremental redundancy, an algorithm is followed that produces values that are usedas Redundant Version (RV) values. This means that a few bits would be punctured starting fromthese positions so that the data rate could be matched with the soft buffer size.

4.2 Spatial Multiplexing

This is a completely logical module. Data is transmitted in packets and these packets are in the formof frames. The LTE Standard defines frames to be of 10ms time duration. The structure of a framedepends on the mode of transmission adopted by the base station (eNodeB) which can either beTime Division Duplexing (TDD) or Frequency Division Duplexing (FDD).

In TDD mode, the uplink and downlink channels function over the same bandwidth and with thesame centre frequency but are time-duplexed whereas in FDD mode, the uplink and downlink channels function over different centre frequencies. Keeping in mind that the modulation schemefollowed is OFDM, each slot comprises of OFDM symbols that cover the total bandwidth availableand, based on the length of the Cyclic Prefix, there are 6 or 7 such columns of symbols that make upa slot of 0.5ms. In fact each slot is further divided into Resource Blocks that are 0.5ms in durationand are 12 carriers wide.Figure 4.10 gives a clear picture of the slot structure.

As specified earlier, data is always associated with its corresponding channel. So the UE needs toknow the channel which the received (encoded) data belongs to so that it could process it

accordingly. Hence LTE Standard specifies fixed positions for the data from different channels ontothe frame. To illustrate this a few examples are used. Refer to Figure 4.11.

So the function of this module is to assign frequency carrier tags to the data based on the tags whichrepresent the channel it belongs to. This is done so that data could be assigned the same frequency during the modulation phase.



Figure 4.10: Slot Structure

Figure 4.11 (a): Position of Synchronization Signals



Figure 4.11 (b): Position of Physical Broadcast Channel

4.3 Transmit Diversity

This module is responsible for facilitating MIMO (Multiple In Multiple Out) scheme which meansthat the transmitter and receiver can have multiple antennas for transmission and reception. Fordownlink transmissions, UEs feedback downlink channel quality information back to the eNode-B.Using a channel quality sensitive scheduler such as proportional fair scheduler, eNode-B can serve aUE on time-frequency resources where it is experiencing the best conditions. At high UE speeds,the channel quality feedback becomes unreliable due to feedback delays. When reliable channelquality estimates are not available at eNode-B, channel-sensitive scheduling becomes infeasible.Under these conditions, it is desired to average out the channel variations by all possible means.Moreover, the channel sensitive scheduler has to wait for the right (good) channel conditions when aUE can be scheduled. This introduces delays in packet transmissions.

Various sources of diversity can be used in an OFDM system. This includes time-diversity,frequency diversity, receive diversity and transmit diversity. The time diversity can, however, beexploited in the LTE system by using hybrid ARQ where retransmissions are spaced approximately 8ms apart at the expense of additional transmission delay. The frequency diversity in the LTEsystem can be exploited by scheduling transmissions over distributed resources. In the uplink slot-level (0.5 ms half subframe), hopping where the transmission is hopped at two frequencies within a



1 ms subframe is used. The transmit diversity provides another additional source of diversity foraveraging out the channel variation either for operation at higher UE speeds or for delay sensitiveservices at both low and high UE speeds.

Figure 4.12: Transmit Diversity

The step-by-step process that is employed before the modulation of data is shown in Figure 4.13.

Figure 4.12: Downlink Transmission Chain

The data is first scrambled using the sc rambler which we had come across in the Encoder module.Then these scrambled bits are mapped to complex modulation symbols by the modulation mapper .



Since MIMO scheme has to be implemented, there are generally 2 or 4 ports at the transmitter. Todistribute data among these ports, it is first mapped to different layers that correspond to theseantenna ports. This is better shown with the help of Figure 4.13

Figure 4.13: Layer Mapping

P re c oding can be seen as a way to improve the spectral efficiency. The incoming bits are multipliedwith a two-dimensional matrix of complex values. The following illustration explains the method.

Consider y (k)(i) to be the output bit stream for the k th antenna port with total number of antennaports to be P.

For the case of 2 antenna ports transmit diversity, the output of the precoding operation is :



RE Mapping is the next block to be executed after precoding. This is where data is assigned aparticular frequency carrier as per the decision taken during spatial multiplexing.

Figure 4.14: RE Mapping in the case of two antennas

Figures 4.15 and 4.16 show the outputs of the precoding and RE mapping for the case of four-portantennas. This is the true implementation of transmit diversity of type Space Frequency Block Coding Frequency Shift Time Diversity (SFBC-FSTD).

Figure 4.15: Precoding for 4-port antenna



Figure 4.16: RE Mapping for 4-port antenna

4.4 OFDM

Orthogonal Frequency Division Multiplexing is a modulation scheme and it is considered the mostrobust signaling scheme. It is different from normal Frequency Division Multiplexing in the way thatthe frequency carriers in normal FDM are spaced apart with guard carriers between them where asin OFDM, the frequency carriers are placed very close to each other but these carriers areorthogonal to each other, thus allowing minimum interference. The mathematical representation of the output signal can be written as:

This equation is very similar to the equation for the calculation of Inverse Fast Fourier Transform.Hence a convenient way has been devised for the modulation of data by OFDM scheme. The data isfirst converted to parallel bit streams and then IFFT is calculated for every set of parallel bits. Cyclicprefix is added to induce continuity. These three modules are implemented under the OFDMmodule.

Figure 4.17: OFDM module



Figure 4.18: OFDM modulation

4.5 RF

This module implements the final stage of the transmission cycle. The data from the OFDM moduleis first converted from digital to analog low pass filter. It is then up converted and passed to thepower amplifier.

Figure 4.19: RF module

4.6 Miscellaneous

Apart from the above mentioned modules there are two more functions to be considered that are :

y Scheduling: This involves scheduling of data for different UEs and amongst different

channels to the same UE too. Although this is done in MAC Layer it is crucial to have arecord of the scheduling in the PHY Layer too.

y Closed loop feedback information: This is the data that is first received in uplink thatinforms the base station about the channel conditions that the UE is experiencing and thusdetermines the values to be used for precoding.



Architecture Overview



References

[1] LTE for 4G Mobile Broadband by Farooq Khan.

[2] LTE The UMTS Long Term Evolution From Theory to Practice by Stefania Sesia.[3] 3GPP Specification TS 36.201.

[4] 3GPP Specification TS 36.211






Development of LTE Network

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