Implementation of an LTE software defned radio for the ORCIP testbed

Universidade de Aveiro Departamento deElectr´onica, Telecomunica¸c˜oes e Inform´atica 2020

Sim˜

ao Pedro

Veiga de Matos

Implementa¸

c˜

ao de um R´

adio Definido por Software

LTE para a Infraestrutura ORCIP

Implementation of an LTE Software Defined Radio

for the ORCIP Testbed

Universidade de Aveiro Departamento deElectr´onica, Telecomunica¸c˜oes e Inform´atica 2020

Sim˜

ao Pedro

Veiga de Matos

Implementa¸

c˜

ao de um R´

adio Definido por Software

LTE para a Infraestrutura ORCIP

Implementation of an LTE Software Defined Radio

for the ORCIP Testbed

Disserta¸c˜ao de Mestrado apresentada `a Universidade de Aveiro, para obten¸c˜ao do grau de Mestre em Engenharia Eletr´onica e de Telecomu-nica¸c˜oes, sob orienta¸c˜ao do Professor Doutor Paulo Monteiro e Doutor Abel Lorences Riesgo

o j´uuri / the jury

presidente / president Professor Doutor Armando Humberto Moreira Nolasco Pinto

Professor Associado da Universidade de Aveiro (por delega¸c˜ao da Reitora da Uni-versidade de Aveiro)

vogais / examiners committee Professor Doutor Paulo Miguel Nepomuceno Pereira Monteiro

Professor Associado da Universidade de Aveiro (Orientador)

Professor Doutor Fernando Jos´e da Silva Velez

agradecimentos / acknowledgements

Em primeiro lugar, um agradecimento especial aos meus pais, irm˜as e av´os por todo o apoio, motiva¸c˜ao e confian¸ca que depositaram em mim ao longo deste percurso acad´emico.

Quero agradecer aos meus orientadores, Professor Doutor Paulo Monteiro, Doutor Abel Riesgo e Doutor Fernando Guiomar por todo o acompan-hamento, apoio e assistˆencia ao longo deste trabalho e que levaram ao sucesso do mesmo.

Agrade¸co aos meus amigos, que sempre ao longo deste percurso me acom-panharam com apoio e motiva¸c˜ao.

Por fim ao Departamento de Eletr´onica, Telecomunica¸c˜oes e Inform´atica, Instituto de Telecomunica¸c˜oes e Universidade de Aveiro por proporcionarem todas as condi¸c˜oes de trabalho necess´arias para a realiza¸c˜ao deste trabalho e de todo o percurso acad´emico.

This work has been hosted by Instituto de Telecomunica¸c˜oes Aveiro and partially supported by the European Regional Development Fund (FEDER), through the Regional Operational Programme of Centre (CEN-TRO 2020) of the Portugal 2020 framework, provided by projects OR-CIP (CENTRO-01-0145-FEDER-022141), 5GO 0247-FEDER-024539), SOCA (CENTRO-01-0145-FEDER-000010), RETIOT (POCI-01-0145-FEDER-016432) and LandMark (POCI-01-0145-FEDER-031527).

Palavras-Chave LTE, SDR, ORCIP

Resumo A constante evolu¸c˜ao das comunica¸c˜oes levou ao desenvolvimento de no-vas tecnologias como o LTE e sistemas SDR. Esta disserta¸c˜ao apresenta o desenvolvimento de um sistema SDR baseado em LTE para o banco de testes do ORCIP. Um estudo detalhado das tecnologias e normas do LTE ´

e primeiramente apresentado para garantir uma completa interpreta¸c˜ao do t´opico. Subsequentemente, usando MATLAB, um transmissor e recetor LTE configur´aveis s˜ao projetados tendo em conta as normas correspondentes. A funcionalidade do transmissor e recetor inclui a gera¸c˜ao e descodifica¸c˜ao de formas de onda LTE com a escolha de m´ultiplos parˆametros como largura de banda, ordem de modula¸c˜ao, n´umero de subframes, entre outros. Com estes m´odulos ´e tamb´em realizada uma implementa¸c˜ao de agrega¸c˜ao de portadoras. Para completar o sistema SDR, um oscilosc´opio e um gerador de sinais arbitr´ario foram conectados para verificar os m´odulos LTE imple-mentados. Os sinais LTE gerados s˜ao carregados para o AWG e as formas de onda capturadas pelo oscilosc´opio s˜ao processadas com o recetor LTE desenvolvido. ´E realizado um teste baseado em medi¸c˜oes de EVM para ambos os cen´arios de portadora ´unica e agrega¸c˜ao de portadoras. Os resul-tados obtidos confirmam um bom desempenho para ambos os cen´arios de portadora ´unica e agrega¸c˜ao de portadoras, provando assim que o sistema implementado pode ser aplicado para sistemas de banco de testes; p. ex. banco de testes do ORCIP.

Keywords LTE, SDR, ORCIP

Abstract The constant evolution in communications has lead to the development of new technologies such as LTE and SDR systems. This dissertation presents the development of a LTE-based SDR system for the ORCIP testbed. A detailed study of LTE technologies and standards is firstly presented, in or-der to ensure a complete unor-derstanding of the topic. Subsequently, using MATLAB, a configurable LTE transmitter and receiver are designed while taking into the corresponding standarts. The functionally of the transmitter and receiver include generating and decoding LTE waveforms with multi-ple parameter choices such as bandwidth, modulation order, number of subframes, among others. With these modules a carrier aggregation imple-mentation is also performed. To complete the SDR system, an oscilloscope and an arbitrary waveform generator were connected to verify the imple-mented LTE modules. The generated LTE signals are loaded to the AWG and the waveforms captured by the oscilloscope are processed with the de-veloped LTE receiver. Test based on EVM measurements are performed for both single carrier and carrier aggregation scenarios. The obtained results confirms a good performance for both single carrier and carrier aggregation, which proves that the system implementation can be applied to testbed sys-tems; e.g. ORCIP testbed.

Contents

List of Figures vi

List of Tables vii

Acronyms xii

1 Introduction 1

1.1 Background and Motivation . . . 1

1.2 Objectives . . . 1

1.3 Document Structure . . . 2

2 LTE - Long Term Evolution 5 2.1 Introduction . . . 5 2.1.1 Basic Technologies . . . 5 2.2 OFDM . . . 6 2.2.1 OFDMA . . . 10 2.2.2 SC-FDMA . . . 10 2.3 Multi-Antenna . . . 11 2.3.1 Receiver Diversity . . . 11 2.3.2 Transmit Diversity . . . 12

2.3.2.1 Closed Loop Transmit Diversity . . . 12

2.3.2.2 Open Loop Transmit Diversity . . . 13

2.3.3 Beamforming . . . 13

2.3.4 Spatial Multiplexing . . . 14

2.3.5 Downlink Transmission Modes . . . 15

2.4 Carrier Aggregation . . . 16

2.5 Physical Layer . . . 17

2.5.1 Frame structure . . . 17

2.5.1.1 Type 1: FDD Frame Structure . . . 17

2.5.1.2 Type 2: TDD Frame Structure . . . 18

2.5.1.3 Type 3: LAA Frame Structure . . . 19

2.5.2 Resource Grid . . . 19

2.5.3 Bandwidth Configuration . . . 19

2.5.4 Physical Signals and Channels . . . 21

2.5.4.1 Downlink Physical Channels . . . 22

2.5.4.2 Downlink Physical Signals . . . 26

2.5.4.3 Uplink Physical Channels . . . 30

2.5.4.4 Uplink Physical Signals . . . 33

2.5.4.6 Physical Channel Processing . . . 35

2.6 Access Procedures . . . 36

3 Architecture and Development 39 3.1 MATLAB LTE Toolbox . . . 39

3.2 Transmitter . . . 39

3.2.1 Resource Grid . . . 41

3.2.2 Synchronization Signals PSS and SSS . . . 41

3.2.3 Reference Signals . . . 41

3.2.4 Physical Broadcast Channel (PBCH) . . . 41

3.2.5 Physical Control Format Indicator Channel (PCFICH) . . . 42

3.2.6 Physical Downlink Control Channel (PDCCH) . . . 42

3.2.7 Physical Hybrid ARQ Indicator Channel (PHICH) . . . 42

3.2.8 Physical Downlink Shared Channel (PDSCH) . . . 43

3.2.9 OFDM Generation . . . 43 3.2.10 Transmitter Simulation . . . 43 3.3 Channel . . . 48 3.4 Receiver . . . 48 3.4.1 Synchronization . . . 49 3.4.2 OFDM Demodulation . . . 51 3.4.3 Channel Estimation . . . 51 3.4.4 Equalization . . . 51

3.4.5 Subframe Channel Estimation . . . 51

3.4.6 Physical Broadcast Channel (PBCH) decode . . . 51

3.4.7 Physical Control Format Indicator Channel (PCFICH) decode . . . . 52

3.4.8 Physical Downlink Control Channel (PDCCH) decode . . . 52

3.4.9 Physical Downlink Shared Channel (PDSCH) decode . . . 52

3.4.10 Compute EVM . . . 52

3.4.10.1 Receiver Simulation . . . 54

3.5 Carrier Aggregation . . . 55

3.5.1 Transmitter Side . . . 57

3.5.1.1 Component Carriers (CCs) Generation . . . 57

3.5.1.2 Carrier Aggregation Configuration . . . 57

3.5.1.3 Oversampling Rate Calculation . . . 59

3.5.1.4 Waveform Generation . . . 59

3.5.2 Receiver Side . . . 59

3.5.2.1 Demodulation and Filtering the selected Component Carrier 59 3.5.2.2 Decoding each Component Carrier . . . 59

3.5.3 Carrier Aggregation Simulation . . . 59

4 Tests and Results 63 4.1 Tests Setup . . . 63

4.2 Experimental Test Settings . . . 64

4.2.1 Single Carrier . . . 65

4.2.1.1 Reference Simulations . . . 66

4.2.2 Carrier Aggregation . . . 66

4.3 Preliminary Experimental Results . . . 69

4.3.1 Results . . . 69

4.4 Single Carrier . . . 73

4.5 Carrier Aggregation . . . 74

5 Conclusions and Future Work 79 5.1 Future Work . . . 80

List of Figures

1.1 Basic SDR receiver and transmitter . . . 2

2.1 LTE releases evolution . . . 5

2.2 Division in subcarriers . . . 6

2.3 Spectrum of a single modulated OFDM subcarrier . . . 7

2.4 Spectrum of multiple OFDM subcarriers . . . 8

2.5 CP insertion . . . 8

2.6 Basic OFDM modulation process . . . 9

2.7 OFDM grid representation . . . 9

2.8 OFDM and OFDMA subcarrier allocation . . . 10

2.9 Comparison of OFDMA and SC-FDMA . . . 11

2.10 Reduction in fading by the use of a diversity receiver . . . 12

2.11 Beamforming . . . 13

2.12 2 x 2 antenna configuration . . . 14

2.13 Types of carrier aggregation . . . 17

2.14 Type 1 FDD frame structure . . . 18

2.15 Type 2 TDD frame structure . . . 18

2.16 Resource grid for uplink (a) and downlink (b) . . . 20

2.17 Channel bandwidth and transmission bandwidth . . . 21

2.18 Control and data region within a subframe . . . 23

2.19 Representation of the downlink physical channels allocation within a subframe 23 2.20 Mapping of downlink CRS for four antenna ports (normal CP) . . . 28

2.21 Mapping of downlink UE-Specific Reference Signal for antenna ports 7, 8, 9 and 10 (normal CP) . . . 29

2.22 Possible CSI-RS positions . . . 30

2.23 Mapping of PRS . . . 31

2.24 Mapping of MBSFN reference signals (extended cyclic prefix, ∆f = 15kHz) . 31 2.25 Illustration of allocated uplink physical channels and signals . . . 33

2.26 Random access preamble . . . 33

2.27 Transport channel processing steps . . . 34

2.28 Transport channel processing steps . . . 35

3.1 Transmitter processing steps . . . 40

3.2 Subframe . . . 46

3.3 Frame (10 subframes) . . . 46

3.4 Waveform generated . . . 47

3.5 Spectrum of the waveform . . . 47

3.6 Channel processing steps . . . 48

3.8 Receiver processing steps . . . 50

3.9 EVM measurement representation . . . 53

3.10 Reference point for EVM measurement . . . 53

3.11 Received grid . . . 54

3.12 Equalized grid . . . 55

3.13 Constellation PDSCH, (orange dots - reference symbols, blue dots - received symbols) . . . 56

3.14 EVM for each subframe for PDSCH . . . 56

3.15 Carrier aggregating processing steps . . . 57

3.16 Carrier aggregation configurations . . . 58

3.17 Three CCs aggregated . . . 61

3.18 Original aggregated signal and the CC1 filtered . . . 61

4.1 Up and Down converted basic process . . . 64

4.2 Test system setup . . . 64

4.3 Equipment setup in the laboratory . . . 65

4.4 EVM per subframe for 20 MHz with 16QAM modulation . . . 70

4.5 Average EVM per attempt for 20 MHz with 16QAM modulation . . . 71

4.6 Average EVM per modulation scheme for 20 MHz . . . 71

4.7 Spectrum of one received waveform . . . 72

4.8 Spectrum of one received waveform with the carrier on 800 MHz . . . 73

4.9 Constellation of 20 MHz 256QAM of PDSCH symbols for one subframe (ref-erence symbols - orange, received symbols - blue) . . . 74

4.10 Spectrum of carrier aggregation with 5 CCs with the carrier on 900 MHz . . 75

4.11 Constellation of CC13 256QAM of PDSCH symbols for one subframe (reference symbols - orange, received symbols - blue) . . . 77

List of Tables

2.1 Transmission Modes . . . 15

2.2 Carrier aggregation bandwidth classes . . . 16

2.3 Uplink-downlink configurations . . . 19

2.4 RB parameters for downlink and uplink . . . 19

2.5 Bandwidth configurations . . . 21

2.6 Downlink physical channels and signals . . . 22

2.7 Uplink physical channels and signals . . . 22

2.8 Antenna ports defined for each reference signal . . . 32

3.1 Transmitter parameteres overview . . . 44

3.2 Values selected . . . 44

3.3 EVM requirements . . . 54

3.4 Parameters for the CCs . . . 60

3.5 Calculations results . . . 60

4.1 Waveform parameters . . . 66

4.2 Single carrier reference EVM values . . . 66

4.3 Bandwidths and CCs selected for carrier aggregation . . . 67

4.4 CCs EVM reference values for 45 MHz (10+15+20 MHz) . . . 67

4.5 CCs EVM reference values for 60 MHz (3×20 MHz) . . . 68

4.6 CCs EVM reference values for 100 MHz (5×20 MHz) . . . 68

4.7 CCs EVM reference values for 300 MHz (15×20 MHz) . . . 68

4.8 OSC parameters . . . 69

4.9 AWG parameters . . . 70

4.10 EVM results for each bandwidth and modulation . . . 72

4.11 EVM results for each bandwidth and modulation . . . 73

4.12 EVM results for 60 MHz (3×20 MHz) . . . 75

4.13 EVM results for 100 MHz (5×20 MHz) . . . 76

4.14 EVM results for 45 MHz (10+15+20 MHz) . . . 76

Acronyms

1G First Generation 2G Second Generation 3G Third Generation

3GPP 3rd Generation Partnership Project 4G Fourth Generation

5G Fifth Generation ACK Acknowledgement

ADC Analog-Digital Converter ARQ Automatic Repeat Request AWG Arbitrary Waveform Generator BCH Broadcast Channel

BPSK Binary Phase-Shift Keying CA Carrier Aggregation

CC Component Carrier

CCE Control Channel Elements CFI Control Format Indicator CP Cyclic Prefix

CRC Cyclic Redundancy Check CRS Cell-Specific Reference Signals CSI Channel State Information

CSI-RS Channel State Information Reference Signal DAC Digital-Analog Converter

DFT-S OFDM Discrete Fourier Transform Spread OFDM DL-SCH Downlink Shared Channel

DM-RS Demodulation Reference Signals ECCE Enhanced Control Channel Elements eNB Evolved NodeB

EPA Extended Pedestrian A model

EPDCCH Enhanced Physical Downlink Control Channel EREG Enhanced Resource Element Group

EVM Error Vector Magnitude FDD Frequency Division Duplex FEC Forward Error Correction FFT Fast Fourier Transform FIR Finite Impulse Response

FPGA Field Programmable Gate Array HI Hybrid ARQ Indicator

ICI Inter-Carrier Interference

IFFT Inverse Fast Fourier Transform IoT Internet of Things

ISI Inter-Symbol Interference IT Instituto de Telecomunica¸c˜oes LAA License Assisted Access LTE Long Term Evolution

MBMS Multimedia Broadcast Multicast Service

MBSFN Multicast-Broadcast Single Frequency Network MCH Multicast Channel

MIB Master Information Block

MISO Multiple Inputs Single Output MMSE Minimum Mean Square Error MU-MIMO Multiple User MIMO NACK Negative Acknowledgement

OFDM Orthogonal Frequency Division Multiplexing OFDMA Orthogonal Frequency Division Multiple Access

ORCIP Optical Radio Convergence Infrastructure for Communications and Power Deliver-ing

OTDOA Observed Time Difference of Arrival PAPR Peak to Average Power Ratio

PBCH Physical Broadcast Channel

PCFICH Physical Control Format Indicator Channel PCH Paging Channel

PDCCH Physical Downlink Control Channel PDSCH Physical Downlink Shared Channel PHICH Physical Hybrid ARQ Indicator Channel PMCH Physical Multicast Channel

PMI Precoder-Matrix Indicator

PRACH Physical Random Access Channel PRB Physical Resource Block

PRS Positioning Reference Signal PSS Primary Synchronization Signal PUCCH Physical Uplink Control Channel PUSCH Physical Uplink Shared Channel QAM Quadrature Amplitude Modulation QPSK Quadrature Phase-Shift Keying RB Resource Block

RE Resource Element

REG Resource Element Group RF Radio Frequency

RI Rank Indicator

RIV Resource Indication Value RMS Root Mean Square

RNTI Radio Network Temporary Identifier RV Redundancy Version

SAE System Architecture Evolution

SC-FDMA Single Carrier Frequency Division Multiple Access SDR Software Defined Radio

SFN System Frame Number SIB System Information Block SIMO Single Input Multiple Outputs SNR Signal to Noise Ratio

SR Sampling Rate

SRS Sounding Reference Signals SSS Secondary Synchronization Signal SU-MIMO Single User MIMO

TDD Time Division Duplex

TDMA Time Division Multiple Access TTI Transmission Time Interval UE User Equipment

UL-SCH Uplink Shared Channel VRB Virtual Resource Block ZF Zero Forcing

CHAPTER

1

Introduction

1.1

Background and Motivation

Since primitive times, communication is essential to humans. Human evolution is directly linked to more and more advanced ways of communication. Technologic developments have led to a connected world specially in mobile communications, which are now widely spread and used by milions of devices. The evolution of mobile communications can be divided in four generations, First Generation (1G) being an analog mobile system, Second Generation (2G) the first digital mobile system, followed by Third Generation (3G) capable of broadband data and Fourth Generation (4G) also referred as Long Term Evolution (LTE) providing high mobile broadband data. This evolution continues each day with the demand for more features, high data rates and new use cases leading to the next new generation - Fifth Generation (5G) [1].

This steadily increasing demand leads to a field of high interest, investigation and projects to further improve the technology. One of that projects, developed by Instituto de Telecomu-nica¸c˜oes (IT) and referred as Optical Radio Convergence Infrastructure for Communications and Power Delivering (ORCIP) consists on a testbed, which provides a platform infrastruc-ture for testing, development and simulation for different scenarios and taking in count every layer from physical to applications level [2].

One key technology to improve the ORCIP development is Software Defined Radio (SDR). SDR provides an efficient and flexible platform for research enabled by the progess in elec-tronics and digital signal processing. SDR systems allow to implement components, which are normally implemented in hardware, in software [3]. This approach allows a more scalable implementation. In a simplied way an SDR system can be represented as in Figure 1.1.

With this SDR technology, radio capabilities can be implemented in software on computers or embedded systems increasing the available options. Also testing and improve new scenar-ios with relative low cost are possible, allowing to further developments on communication systems.

1.2

Objectives

The aim of this document is to development and implement an SDR system based on LTE for ORCIP testbed, that is a system capable of generating and processing LTE waveforms with parameters choices such as modulation format and bandwidth defined by the user. To achieve this global target, this Thesis has the following objectives:

Figure 1.1: Basic SDR receiver and transmitter

physical layer to ensure full compreension of the topic;

• Using MATLAB and its LTE Toolbox, implement a configurable transmitter and re-ceiver based on the standarts and also capable of performing carrier aggregation. This software implementation can be seen as the digital processing stage on a SDR system as in Figure 1.1;

• Connect the MATLAB implementation to a hardware setup capable of transmitting and receiving the generated waveforms. The hardware can be seen as the Radio Frequency (RF) front end with analog/digital converts in Figure 1.1. With the overall SDR system completed, some performance tests can be performed to test the quality of the system.

1.3

Document Structure

The document is divided in five chapters and a brief description of each one contents are provided:

• Chapter 1 (Introduction): Brief description of the scope of this document as well as the evolution of mobile communications, new paradigm with SDR technologies and the ORCIP main project where this document is inserted;

• Chapter 2 (LTE - Long Term Evolution): Detailed description of LTE technologies particularly on physical layer. A description based on the standarts from the base transmission scheme to physical channels and procedures for both downlink and uplink scenarios;

• Chapter 3 (Architecture and Development): Description of the design and implemen-tation in MATLAB of the transmitter, receiver and carrier aggregation with the stages and procedures needed;

• Chapter 4 (Tests and Results): Describes the hardware equipment added to complete the overall SDR system. This hardware allows to perform tests on the generated waveform in a back to back configuration;

• Chapter 5 (Conclusion and Future Work): In this last chapter, some conclusions of the overall work developed are given and some future works are indicated.

CHAPTER

2

LTE - Long Term Evolution

This chapter provides a description of LTE, focus on physical layer. First the basic trans-mission scheme is described, followed by multi-antenna techniques and carrier aggregation. The fundamental physical time-frequency structure of LTE transmissions for both downlink and uplink is discussed. Then a detailed description of the downlink and uplink functional-ity, including physical channels and signals and transport and physical channel processing is provided.

2.1

Introduction

The LTE was developed by 3rd Generation Partnership Project (3GPP) to be the evolution of the 3G, and hence the reason to be referred as 4G. However, the two generations have little in common. The aim of LTE was to provide a new radio access technology based on packet-switched data communications, capable of providing high data rates, high spectral efficiency, low latency and frequency flexibility. Also, a evolution of the network architecture, including the core network was needed, leading to the creation of the System Architecture Evolution (SAE). LTE is specified in releases, being added new features in each new release. The first release of LTE was Release 8, which defined the basic technologies and requirements, and is currently on Release 15 which also includes the new 5G radio access. Some releases include important improvements for example Release 10 which introduces the LTE Advanced and Release 13 which introduces LTE Advanced Pro(Figure 2.1) [4, 5, 6].

LTE

Rel-8 Rel-10 Rel-13

LTE Advanced LTE Advanced Pro

Figure 2.1: LTE releases evolution

2.1.1 Basic Technologies

Since the first release (Release 8), LTE supports frequency flexibility by supporting both unpaired spectrum Time Division Duplex (TDD) and paired spectrum Frequency Division Duplex (FDD) duplex schemes for downlink and uplink. Downlink is the transmission from

a Evolved NodeB (eNB) (base station) to an User Equipment (UE) (device) and uplink is the opposite, that is a transmission from an UE to the eNB. Also a wide operating bands are defined, from less than 1 GHz to almost 6 GHz and are described in 3GPP standart TS 36.101 [7]. The basic transmission scheme used is Orthogonal Frequency Division Multiplexing (OFDM) and supports bandwidths up to 20 MHz. Also supports multi-antenna transmission, improving the performance of the system. With the futher releases, new features such as Carrier Aggregation (CA) were added. In CA multiple carriers can be aggregated, increasing the transmission bandwidth and hence the data rates. Another added feature was License Assisted Access (LAA), allowing to combine licensed spectrum with unlicensed spectrum, which can be complemented with CA to offer higher data rates. Among other features, LTE also introduced new use cases, with support for Internet of Things (IoT) applications as Machine Type Communications. One of the results of this evolution was in data rates. In Release 8, support for up to 300 Mbps in downlink and 75 Mbps for uplink was introduced and after some releases the data rates supported can be up to several Gbps [1, 6].

2.2

OFDM

OFDM is the transmission scheme used in LTE. The concept of OFDM was proposed and investigated during the 1960s and 1970s and nowadays is widely used in a lot of applications. The basic principle of OFDM is to divide the available bandwidth into narrowband parallel channels referred to as subcarriers and transmit information on these parallel channels (Figure 2.2) at a lower data rate [8].

Figure 2.2: Division in subcarriers [9]

The overall low rate subcarriers together, provide data rates identical to single carrier schemes using the same bandwidth [4]. Each of these subcarriers support any modulation, in this case, such as Quadrature Phase-Shift Keying (QPSK), 16Quadrature Amplitude

Modu-lation (QAM), 64QAM and 256QAM. The number of subcarriers depends on the subcarrier spacing ∆f and available bandwidth. The subcarrier spacing ∆f is associated with the symbol duration of each subcarrier T by:

∆f = 1

T (2.1)

In LTE, the standart subcarrier spacing ∆f is 15 KHz, so the symbol duration is 66.7 µs. Also, is specified a subcarrier spacing of 7.5 KHz, however is partly used [5, 10].

OFDM uses rectangular pulse shaping that represents a sinc in frequency domain, Figure 2.3.

f

Figure 2.3: Spectrum of a single modulated OFDM subcarrier A basic OFDM baseband signal x(t) can be expressed as:

x(t) = N −1 X k=0 xk(t) = N −1 X k=0 a(m)_k ej2πk∆f t (2.2)

where x(t) is the kth modulated subcarrier with frequency f = k∆f and a(m)_k is the complex modulation symbol transmitted on the kth subcarrier during the m symbol interval [11]. A more detailed explanation is described in 3GPP standart TS 36.211 [10].

The subcarriers are spaced in a way that the nulls of each subcarrier line up with the center peak of the neighboring subcarriers. This property is call ”orthogonality” and the reason why OFDM is referred as ”Orthogonal Frequency Division Multiplexing”. Orthogonality provides no interference between subcarriers and improves spectral efficiency because don’t need guard bands between the overlapped subcarriers [4, 12]. Figure 2.4 represents the spectrum and orthogonality of multiple OFDM subcarriers.

The orthogonality can be lost due to delay spread in the transmission channel, causing Inter-Symbol Interference (ISI) and inter subcarriers interference (ICI). The term delay spread describes the time difference between the first signal received and the delayed copies of same signal arrived from multiple paths, also called multipath reflections. These reflections can be cause by buildings or other terrain obstacles. The symbols received along a delayed path can interfere with symbols received via a more direct path and causing ISI [13]. To avoid this problem, a guard period called Cyclic Prefix (CP) is used. The CP is inserted in the beginning of an OFDM symbol by copping the last part of the OFDM symbol, so there is not new information added. This insertion increases the duration of the OFDM symbol to

Δf

Figure 2.4: Spectrum of multiple OFDM subcarriers

T + TCP, where TCP is the length of the CP, decreasing the symbol rate. However, in spite

of this rate reduction, the CP insertion is beneficial because makes the signal resistant to multipath dispersion allowing to avoid ISI and ICI as long as the CP duration is longer than the delay spread [11]. This is illustrated in Figure 2.5. With the CP the delayed symbols don’t overlap the symbols received in a more direct path, which means that the signal is sampled when is stable and no other delayed reflections affect, avoiding interference.

CP insertion T+TCP Direct Path Delayed Path TCP T Figure 2.5: CP insertion

A basic OFDM implementation is illustrated in Figure 2.6. The complex modulation symbols for example QAM, Data In in the figure, are mapped to the subcarriers in frequency domain. Then a Inverse Fast Fourier Transform (IFFT) function converts the subcarriers to time domain, the CP is inserted to each waveform and the resulting signal can be modulated and transmitted. In the receiver side, the signal is demodulated and then CP is removed. After, an Fast Fourier Transform (FFT) function convert the signal to frequency domain, then the subcarriers can be detected and the complex symbol can be obtain [8, 12].

A common representation of OFDM is a time-frequency grid illustrated in Figure 2.7, where each unit represent a subcarrier in frequency domain and a symbol in time domain.

In spite of the benefits, OFDM has some few drawbacks. One of them is the sensibility to frequency errors due to oscillators drifts and Doppler shifts and the other is a high Peak to Average Power Ratio (PAPR). As the OFDM symbols are a combination of subcarriers, the added components cause high peaks in time domain, much higher than the average power. This problem decrease the efficiency of the RF amplifiers, as the amplifiers need to handle the peak values leading to a increased power consumption and also affects the Analog-Digital

Channel Serial to Parallel Parallel to Serial

IFFT CP Insertion CP Removal FFT

Data In Data Out

Figure 2.6: Basic OFDM modulation process

Fr

_eq

u

_en

cy

Time

Converter (ADC) and Digital-Analog Converter (DAC) dynamic range. PAPR is a major problem in uplink, as the available power is limited [9, 13].

2.2.1 OFDMA

The standard OFDM discussed in the previous section assumed that the subcarriers were fixed to one receiver. However, in practice, LTE for downlink transmissions uses a variance of OFDM, who allow to transmit to multiple users. This is called Orthogonal Frequency Division Multiple Access (OFDMA) and incorporate elements from Time Division Multiple Access (TDMA) who add support for user multiplexing, so different users can be allocated to a specified number of subcarriers for a period of time. This is illustrated in Figure 2.8. The scheduling of subcarriers by user also increases capacity by allocating subcarriers in accordance with the needs of the user. Another advantage is to avoid scheduling subcarriers which suffers from multipath and frequency selective fading, selecting the users with best channel conditions, [4, 5, 11].

Figure 2.8: OFDM and OFDMA subcarrier allocation [4]

2.2.2 SC-FDMA

In LTE uplink, as the devices may be driven by batteries, the power consumption must be efficiency so a high PAPR isn’t suitable. The chosen scheme was Single Carrier Frequency Division Multiple Access (SC-FDMA), that combines the properties of OFDMA and can achieve a low PAPR. The SC-FDMA is similar to OFDMA but with some additional steps. One of them is to use a FFT precoder, which convert the input time signal to frequency domain and the resulting signal is applied to consecutive subcarriers. This process spreads the information across the consecutive subcarriers, reducing the power difference between subcarriers leading to a decreasing in PAPR. Then the result is feed to the IFFT and the process continues similar to OFDMA. A detailed mathematical explanation can be found in 3GPP standart TS 36.211 [10]. Because of this process, SC-FDMA is also called Discrete Fourier Transform Spread OFDM (DFT-S OFDM) [4, 8, 12]. In Figure 2.9 a comparison between OFDMA and SC-FDMA is illustrated. The figure shows, in the case of OFDMA, the symbols transmitted in parallel, each one in a subcarrier. On the other hand, in the case

of SC-FDMA, the symbols are spread across a continuous set of subcarriers, appear to be like a single carrier and hence the name Single Carrier (SC). Also the CP insertion is shown as a gap but is only for an easy representation.

Figure 2.9: Comparison of OFDMA and SC-FDMA [4]

2.3

Multi-Antenna

Multi-antenna techniques were addopted to LTE since the beginning. They can improve the system performance in different ways. LTE supports multiple antennas in base stations and devices, both in downlink and uplink. There are three main antenna techniques which are applied in LTE, with different aims and purposes. The first one, diversity, provides additional surpression against fading by using multiple antennas to collecting more received power. Makes use of path diversity and can be use at the transmitter, receiver or both. The second is called beamforming and combine multiple antennas in the transmitter. These antennas can control the radiated beam, for example to increase coverage in a certain direction. The third one uses both transmitter and receiver antennas to transmit more than one stream of data in parallel and is referred as spatial multiplexing. Sometimes is also referred to as Multiple Inputs Multiple Outputs (MIMO), but this name is a little ambiguous. MIMO refers to the channel, the input and output of the air interface and not the inputs and outputs of the devices. Also, sometimes MIMO can include transmit and received diversity and not only spatial multiplexing [4, 9, 11]. In this section a basic overview about these techniques is given.

2.3.1 Receiver Diversity

The choice of multiple antennas in the receiver side is often used in the uplink but can be used in downlink as well. The antennas in the base station received copies of the signal and try to combine that copies. As these signals can be affected by fading, when combined, the

fades of the signals could align and the combined power is low. This suggests, that the signals experience the same effects. But, if the antennas in the base station are distant enough with each others, the signals received will experience different effects and so, when combined, the fades will not align improving the combined signal [9]. Receiver diversity can also be describe as Single Input Multiple Outputs (SIMO) [4]. The requirements in 3GPP, assume that the receiver has two receiver antennas as baseline [7]. Figure 2.10 illustrate a base station with two receiving antennas, receiving two signals with fading and combining it.

Figure 2.10: Reduction in fading by the use of a diversity receiver [9]

2.3.2 Transmit Diversity

Transmit diversity is similar to receiver diversity but use multiple antennas at the trans-mitter side, also called Multiple Inputs Single Output (MISO). Only requires one receiver antenna, therefore the signals are added in one antenna and may suffer from destructive in-terference. The two possible solutions for this problem are Closed Loop Transmit Diversity and Open Loop Transmit Diversity. These modes are also called codebook-based precoding. A simple explanation can be, the modulation symbols are mapped to layers and then the layers are mapped to antenna ports using a precoder that can be described by a precoder matrix. Layers is a synonymous of stream. Antenna Ports is not the same as a physical antenna, is a logical concept, and can be seen to corresponding to a certain reference signal. Therefore, if multiple antennas transmitted the same reference signal, they correspond to one antenna port [11]. The antenna port definition by LTE can be found in 3GPP standart TS 36.211 [10].

2.3.2.1 Closed Loop Transmit Diversity

This method relies on Cell-Specific Reference Signals (CRS) measurements of the channel, to select the best suitable transmission rank and precoder matrix. The transmission rank is often use to refere the number of layers. The receiver selects the transmission rank and

corresponding precoder matrix and send it to the transmitter, hence the feedback loop. That information is transmitted in Rank Indicator (RI) and Precoder-Matrix Indicator (PMI), respectively. However, the network may not use the RI and PMI provided by the receiver, allowing to choose by itself. Each transmission rank has only a limited number of precoder matrices that can be used, known by codebook. This method introduces delay due to the feedback loop, specially in high mobility scenarios, so the RI and PMI can be outdated when used [9, 11].

2.3.2.2 Open Loop Transmit Diversity

Open loop solves the delay problem of close loop feedback, being a suitable option for high mobility scenarios. This method does not use feedback from the receiver and the precoder matrix chosen is known by the receiver. Open loop processing is similar to closed loop, however the precoding matrix structure is different.

2.3.3 Beamforming

Beamforming, in a basic concept, is a technique in which the antenna beam can be con-trolled and steered to a specific direction. In this way, it’s possible to increase the signal strength at a particular region of interest or a target receiver. In general, this can be achieved by the use of antenna arrays. To steer the beam, different phase shifts can be applied to the signal, and then be applied to multiple antennas, creating constructive interference on a particular direction (forming the beam) and destructive interference in the other directions. In Figure 2.11, is represented a beamforming situation where each beam is steered to a user in different places, avoiding sending power to undesired locations and also improving the transmission to each user [11].

Phase Shifts User 1

User 2

2.3.4 Spatial Multiplexing

Spatial multiplexing make uses of both multiple antennas in transmitter and receiver side by creating multiple parallel transmission layers also called streams on the same time and frequency. This can increase the data rate proportional to the number of layers NL.

Assuming NT transmit antennas and NR receive antennas, the number of parallel layers can

be up to min{NT, NR}. However, with limited bandwidth, the achievable data rates cannot

keep increasing. This can be explained by the channel capacity C:

C = BW log2(1 + SN R) (2.3)

where BW is the bandwidth, and Signal to Noise Ratio (SNR), thus the channel capacity (C) increases with the increase in SNR, when BW is limited. In the case of multiple antennas the receiver SNR can be increased. With multiple parallel NLlayers, each one with NLtimes

lower the SNR, the overall channel capacity is given by: C = BW · log2 Å 1 +NR NL · SN R ã (2.4) Being the number of layers NLdependent of the number of antennas, the channel capacity

is therefore dependent on the number of antennas [11]. In the Figure 2.12, a 2x2 (NT × NR)

configuration, two transmiting antennas and two receiving antennas, is illustrated.

Figure 2.12: 2 x 2 antenna configuration [11]

Each receiver antenna receives the transmissions of each transmit antenna, creating in this case four possible paths between the transmit and receive antennas. Each path can be identified by a coefficient, hR,T, representing the channel amplitude and phase response.

These coefficients make a so called channel matrix H, that can be use by the receiver to recover the transmitted signals. The received signals can be expressed as:

r = H · s + n (2.5) ñr1 r2 ô =ñh1,1 h1,2 h2,1 h2,2 ô ·ñs1 s2 ô +ñn1 n2 ô (2.6) where, r is the received signals, s is the transmitted signals and n is the noise added. As-suming perfect conditions, the received signals can be multiply by the inverse of the channel matrix (H−1) and the transmitted signals can be recovered. In order to receiver estimate the coefficients of the channel matrix, the transmitter sends known reference signals which

allow the receiver to estimate the effects on the signals and calculate the channels coefficients. Note that, the number of receiving antennas NR most be at least the same as the number of

transmitted layers, otherwise it is not possible to calculate the channel matrix H. Also, in the case of bad conditions, spatial multiplexing may not be the suitable choice because requires good SNR [4, 9, 11].

In LTE, up to eight transmission layers are supported for downlink, and four to uplink [1]. It is also usefull to understand the difference between Single User MIMO (SU-MIMO) and Multiple User MIMO (MU-MIMO), both based on spatial multiplexing. In SU-MIMO, the transmitter sends to only a single receiver (single user), increasing the data rates to that user. In MU-MIMO, the transmitter sends to multiple receivers (multiple users) on the same time-frequency. However, this not increase the data rate per user as the streams are shared per users but increase the throughput of the transmitter [4].

2.3.5 Downlink Transmission Modes

To support the use of multiple antennas, LTE defines ten different transmission modes, which differ in antenna mapping and what reference signals are used [5], to provide the best benefits to each case. Table 2.1 outlines the transmissions modes, the transmission scheme of each one and also the antenna ports used. Only the transmission mode 1 represents single antenna transmission. A more complete description and explanation of each mode can be found at 3GPP standart TS 36.213 [14].

Transmission

Mode Transmission Scheme

Antenna Ports 1 Single-antenna transmission

0 - 3 2 Transmit diversity

3 Large delay CDD or Transmit diversity 4 Closed-loop spatial multiplexing or

Transmit diversity

5 Transmit diversity or Multi-user MIMO 6 Transmit diversity or Closed-loop spatial

multiplexing using a single transmission layer

7 Single antenna or Transmit diversity 5

8 Single antenna or Dual layer transmission or

Transmit diversity 7 - 8

9 Single antenna or up to Eight layer transmission or

Transmit diversity 7 - 14

10

Single antenna or up to Eight layer transmission or Transmit diversity

(extension of transmission mode 9 for multi-point coordination)

2.4

Carrier Aggregation

With the increasing demand for bandwidth and higher data rates, 3GPP introduce a way to transmit multiple LTE carriers in parallel. This technique is called Carrier Aggregation (CA) and it was introduced in Release 10, with support up to five aggregated carriers that corresponds to a maximum bandwidth of 100 MHz. In general, each carrier is called Com-ponent Carrier (CC) and supports up to 20 MHz of bandwidth. In later releases, CA was extend to 32 carriers allowing a maximum bandwidth of 640 MHz. The CCs aggregated do not need to have the same bandwidth, so is possible to aggregate one CC with 15 MHz to another with 20 MHz. Each CC is based on the physical layer released in Release 8 to assure that older devices can access to them. CA was also motivated to improve spectral efficiency due to the possibility to aggregate noncontiguous carriers in a fragmented available spectrum. In Figure 2.13, the three different possible cases of CA, are illustrated. There are two cases of Intra-band aggregation, that is the CCs are in the same band, one with contiguous CCs and the other with separated CCs. The other case left is Inter-band aggregation, that is, the CCs are in different bands. When released, CA only supported aggregated CCs with the same duplex scheme (TDD or FDD) and in TDD the downlink-uplink configuration must be the same. In later releases, this was improved, CCs with different duplex shemes can be aggre-gated and it was given the possibility of different downlink-uplink configurations in TDD. In CA, there is one Primary CC for downlink and one for uplink on FDD mode and only one for TDD mode. The others carriers are referred as Secondary CCs [5]. In the specifications, there are defined CA configurations that combine the operating bands and bandwidth classes used. The format changes for each one of the three different cases. For example [15]:

• CA 1C indicates intra-band contiguous on operating band 1 and bandwidth class C. • CA 1A 1A indicates intra-band noncontiguous on operating band 1 with one CC on

each side of the intra-band gap and bandwidth class A.

• CA 1A 5B indicates inter-band, on operating band 1 with bandwidth class A and op-erating band 5 with bandwidth class B.

CA bandwidth classes referred above, define the number of CCs that can be aggregated contiguous in a single band and is shown in Table 2.2 [7]. A more detailed information about carrier aggregation can be found in 3GPP standart TS 36.101 [7].

CA Bandwidth Class Number of contiguous CC

A 1 B 2 C 2 D 3 E 4 F 5 I 8

Figure 2.13: Types of carrier aggregation [11]

2.5

Physical Layer

This section describes the LTE physical layer. This is the lower protocol layer, which interact with hardware to provide the data transmission.

2.5.1 Frame structure

LTE supports two duplex schemes, TDD and FDD, each one with different features leading to different frame structures. Downlink and uplink transmissions are organized in frames. In the specifcations, it is used a time unit expressed as:

Ts=

1

15000 × 2048(s) (2.7)

to define the timing. The frame structure is defined in time domain, has a duration of Tf rame = 307200 · Ts = 10 ms and each one is numbered by a System Frame Number (SFN)

which repeats itself from 0 to 1023.

Each frame is divided into 10 equal subframes, each one with a duration of Tsubf rame =

30720 · Ts = 1 ms and are numbered from 0 to 9. Each subframe represents two equal slots

with a duration of Tslot= 15360 · Ts = 0.5 ms [10].

There are three types of frame structures defined: • Type 1 : FDD Frame Structure

• Type 2 : TDD Frame Structure • Type 3 : LAA Frame Structure

2.5.1.1 Type 1: FDD Frame Structure

Figure 2.14 illustrates one FDD frame, with its subframes and slots. As FDD has two carries separated in frequency, one for downlink and other for uplink, there are 10 subframes available for downlink and 10 for uplink [10].

Figure 2.14: Type 1 FDD frame structure [4]

2.5.1.2 Type 2: TDD Frame Structure

In Figure 2.15 one TDD frame is represented. Each frame consists of two half-frames of duration Thalf −f rame= 153600 · Ts= 5 ms with five subframes each.

Figure 2.15: Type 2 TDD frame structure [4]

In TDD, where the downlink and uplink are time-multiplexing over a frame, each subframe can be assigned to downlink or uplink. To performe this distribution between subframes, were specified some uplink-downlink configurations, in which each subframe is assigned to ”D” for downlink, ”U” for uplink and ”S” for a special subframe. The uplink-downlink configurations are described in Table 2.3. In the case of a downlink-to-uplink switch-point of 5 ms, the special subframe exists in both half-frames, in the case of 10 ms only exists in the first half-frame [10]. The special subframe contain three fields, downlink pilot timeslot (DwPTS ), guard period (GP ) and uplink pilot timeslot (UpPTS ). The guard period assures the switching between downlink and uplink and vice-versa. The length of these fields is dependent of the configuration of special subframe and given in 3GPP standart TS 36.211 [10].

Uplink-downlink configuration Downlink-to-Uplink Switch-point periodicity Subframe number 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms D S U U U D D D D D 4 10 ms D S U U D D D D D D 5 10 ms D S U D D D D D D D 6 5 ms D S U U U D S U U D

Table 2.3: Uplink-downlink configurations [10]

2.5.1.3 Type 3: LAA Frame Structure

In LAA Frame Structure, each frame is 10 ms long, with 10 subframes, each one with two slots. The 10 subframes are available for downlink or uplink and their transmissions can start anywhere within a subframe [10].

2.5.2 Resource Grid

The transmitted signal is represented on a time-frequency grid, similar to OFDM, called Resource Grid. The smallest unit on the resource grid, is a Resource Element (RE) and represent one OFDM or SC-FDMA symbol in time domain and a subcarrier in frequency domain. The REs are grouped in Resource Block (RB)s, which has a duration of a slot and is represented by the number of consecutive subcarriers N_scRB and the number of consecutive symbols N_symbDL (downlink) or N_symbU L (uplink), thus N_scRB × N_symb resource elements (RE). In frequency domain each RB corresponds to 180 kHz, which for the selected subcarrier spacing (∆f = 15 KHz), corresponds to 12 subcarriers. In time domain, the number of symbols per slot and thus per RB is dependent of the cyclic prefix length (CP), that can be normal or extended. Table 2.4 represents the various parameters for RB, in function of CP length and subcarrier spacing. Note that, only downlink supports other subcarrier spacings. Therefore, a resource grid, can be represented by N_RBU L× NRB

sc (for uplink) or NRBDL× NscRB (for downlink)

in frequency domain and N_symbU L (for uplink) or N_symbDL (for downlink) symbols in time domain. A example of a resource grid only with a slot is illustrated in Figure 2.16 [4, 10].

Configuration N_scRB N_symbDL N_symbU L Normal Cyclic Prefix ∆f = 15 kHz

12 7 7

Extended Cyclic Prefix

∆f = 15 kHz 6 6

∆f = 7.5 kHz 24 3

-∆f = 1.25 kHz 144 1

-Table 2.4: RB parameters for downlink and uplink

2.5.3 Bandwidth Configuration

LTE defines six different transmission bandwidth configurations, which can be described in MHz or in resource blocks (RB), representing the maximum number of RBs that can be

(a) (b)

transmitted. Note, that the actual occupied bandwidth is defined by the number of RBs. For example for a total bandwidth of 1.4 MHz, represents 6 RBs, which corresponds to 6 × 12 = 72 subcarriers and a bandwidth of 72 × 15 KHz = 1.08 MHz occupied. The restant left bandwidth can be use for guard bands. This is described in Table 2.5 and in Figure 2.17. Also it is important to notice that the subcarrier that coincides with carrier center frequency (DC subcarrier ) is not used for downlink, but is used for uplink, being the carrier center frequency for uplink between two subcarriers [5].

Total bandwidth Number of resource blocks Number of subcarriers Occupied bandwidth Usual guard bands 1.4 MHz 6 72 1.08 MHz 2 × 0.16 MHz 3 MHz 15 180 2.7 MHz 2 × 0.15 MHz 5 MHz 25 300 4.5 MHz 2 × 0.25 MHz 10 MHz 50 600 9 MHz 2 × 0.5 MHz 15 MHz 75 900 13.5 MHz 2 × 0.75 MHz 20 MHz 100 1200 18 MHz 2 × 1 MHz

Table 2.5: Bandwidth configurations [9]

Transmission Bandwidth [RB]

Transmission Bandwidth Configuration [RB] Channel Bandwidth [MHz]

Center subcarrier (corresponds to DC in baseband) is not transmitted in downlink Active Resource Blocks

C h an n e l e d g e C h a n n e l e d g e R e s o u rc e b lo c k

Figure 2.17: Channel bandwidth and transmission bandwidth [10]

2.5.4 Physical Signals and Channels

The physical layer interacts with higher layers by means of transport channels, which transport for example the data to transmit. These transport channels are mapped to physical channels, which are a set of resource elements (REs). Also, there are some physical channels, independent of transport channels and usually carry control information. There are also physical signals, which are a set of resource elements as physical channels, but does not carry information from higher layers and are usually associated with reference signals . Transport channels, physical channels and physical signals exists for downlink and uplink [9, 10, 11].

In Table 2.6 and Table 2.7 the physical channels and signals for downlink and uplink are represented. In the next section, each one will be described.

Downlink

Physical Channels Physical Signals

Physical Downlink Shared Channel Primary Synchronization Signal Physical Broadcast Channel Secondary Synchronization Signal Physical Multicast Channel Cell-Specific Reference Signal Physical Control Format Indicator Channel UE-Specific Reference Signal Physical Downlink Control Channel Positioning Reference Signal Physical Hybrid ARQ Indicator Channel Channel State Information

Reference Signal

Enhanced Physical Downlink Control Channel MBSFN Reference Signal Demodulation Reference Signal for EPDCCH

Table 2.6: Downlink physical channels and signals

Uplink

Physical Channels Physical Signals

Physical Uplink Shared channel Demodulation Reference Signal Physical Uplink Control Channel Sounding Reference Signal Physical Random Access Channel

Table 2.7: Uplink physical channels and signals

2.5.4.1 Downlink Physical Channels

The overall physical channels can be divided in two types, control information and data transmission. The control channels form the Downlink L1/L2 control signaling and contain vital information to properly decode de data transmissions. The L1/L2 indicates that the information comes from Layer 1 and Layer 2. Therefore, the subframe consists in two re-gions, control region and data region as represented in Figure 2.18. The size of control region in time can vary with the information but spans always the whole bandwidth and contains the Physical Control Format Indicator Channel (PCFICH), Physical Hybrid ARQ Indicator Channel (PHICH) and Physical Downlink Control Channel (PDCCH). The data region is the rest of the subframe that is not occupied by the control region and contains the Physical Downlink Shared Channel (PDSCH), Physical Broadcast Channel (PBCH), Enhanced Phys-ical Downlink Control Channel (EPDCCH) and PhysPhys-ical Multicast Channel (PMCH). In the next sections, will be described how the channels are allocated in each region and in Figure 2.19 a representation of their allocation within a subframe.

Figure 2.18: Control and data region within a subframe

2.5.4.1.1 Physical Downlink Shared Channel

The PDSCH is the channel used for data transmission to the users, paging messages, and some remaining system information messages called System Information Block (SIB)s. The data to transmit can be to multiple users and is carried from higher layers (MAC layer) on Downlink Shared Channel (DL-SCH) transport channel. The paging messages are carry on Paging Channel (PCH) transport channel. The data in DL-SCH is transported in transport blocks and can be delivery up to two transport blocks per Transmission Time Interval (TTI), which corresponds to one subframe of 1 ms. The number of transport blocks depends on the transmission mode, specially with spatial multiplexing. Transport block processing will be described in next sections. PDSCH supports QPSK, 16QAM, 64QAM and 256QAM modulation schemes and can be allocated to almost all resource elements (REs) except for the ones allocated to references signal and control. The processing of PDSCH is also dependent on the transmission modes already described [4, 5, 16]. Figure 2.19 shows a representation of a subframe with the PDSCH allocated.

2.5.4.1.2 Physical Broadcast Channel

The PBCH carries the Broadcast Channel (BCH) transport channel, which carries system information on the Master Information Block (MIB). The MIB transports essential informa-tion, specially for initial access, as downlink transmission bandwidth, PHICH duration and resource and system frame number (SFN). It is transmitted within a TTI of 40 ms, so in successive four frames because the BCH is processed every 40 ms. The PBCH always occupy the central 72 subcarriers (6 RBs) of the channel and is transmitted on first subframe of each frame (subframe 0), on slot 1 in the symbols 0,1,2 and 3. Note that, PBCH skips the resource elements (REs) allocated for reference signals and only supports QPSK modulation [4, 5, 16]. Figure 2.19 shows a representation of a subframe with the PBCH allocated.

2.5.4.1.3 Physical Hybrid ARQ Indicator Channel

The PHICH carries the Hybrid Automatic Repeat Request (ARQ) Indicator (HI). The base station (eNB) sends a feedback on HI in the form of Acknowledgement (ACK) (indicator set to ”1”) or Negative Acknowledgement (NACK) (indicator set to ”0”) to the transmitter device UE. This informs if the transmitted uplink data was correctly received or, otherwise, is need to be retransmitted. PHICH is located on downlink control region, where multiple PHICHs are mapped to the same Resource Element Group (REG)s forming PHICH groups. Each group requires three REGs of four resource elements (REs) and a maximum of eight ACK/NACK can be multiplexed, enabling feedback to eight devices in a single group. To separate the multiple PHICHs in the same group are used different orthogonal sequences. A representation of an allocated PHICH is illustrated in Figure 2.19. PHICH can have two durations on a subframe, normal which occupies the first symbol or extended that can occupy up to three symbols. The modulation scheme supported is Binary Phase-Shift Keying (BPSK) [4, 9, 16].

2.5.4.1.4 Physical Control Format Indicator Channel

The PCFICH transports the number of OFDM symbols used for PDCCH in a subframe, thus it is fundamental to be correctly decoded, otherwise the decoding of the control

informa-tion and corresponding data would be unachievable. It contains the Control Format Indicator (CFI) indicating the number of OFDM symbols used by PDCCH, given by the base station, and can take the values of 1, 2 or 3. For bandwidths N_RBDL > 10 the number of OFDM sym-bols for PDCCH is the same as CFI, otherwise the symsym-bols are given by CF I + 1 =2, 3 or 4. The PCFICH is transmitted on every subframe in the control region on first symbol and is mapped to four REGs (16 resource elements REs). The location of the REGs in frequency domain is relative to cell identity, to minimize inter cell interference. A representation of an allocated PCFICH is illustrated in Figure 2.19. It only supports QPSK modulation [4, 9, 16]. 2.5.4.1.5 Physical Downlink Control Channel

PDCCH carries the Downlink Control Information (DCI), which includes transport format information, resource allocation (including PDSCH resource allocation) and transmit power control. Depending on the information, DCI can have different formats also related to the transmission modes. A description of each DCI format can be found in 3GPP standart TS 36.212 [17].

The number of OFDM symbols allocated for PDCCH is given by the CFI as described before and it is transmitted in the control region. PDCCH is mapped in Control Channel Elements (CCE)s and each CCE consist of nine REGs. A representation of an allocated PDCCH is illustrated in Figure 2.19. The number of CCEs per PDCCH can vary and they are grouped in one, two, four or eight consecutive CCEs also referred as aggregation level, leading to different PDCCH formats. However, as the number of CCEs is not indicated, the UE needs to blind decode the number. To reduce the number of blind attempts, LTE specifies two spaces for PDCCH, referred as device-specific search space and common search space. In the case of device-specific search space, the search space is only valid for one specified device (UE) in contrast to common search space,where the search space is addressed to multiple devices (UE). To assure multiple devices, a Radio Network Temporary Identifier (RNTI) can be assigned to each device and the DCI messages intended for each device can be coded with the specific RNTI. There can be multiples PDCCH transmissions on a subframe and only supports QPSK modulation[4, 5, 16].

2.5.4.1.6 Enhanced Physical Downlink Control Channel

Although the PDCCH is suitable for most situations, with the increasing features in LTE, became limited and hence it was introduced the EPDCCH. In spite of transmitting both the same information there are two main differences between the channels. The EPDCCH is located in the data region and occupied a limited bandwidth, unlike the PDCCH which is located in the control region and occupies all the bandwidth. A representation of an allocated EPDCCH is illustrated in Figure 2.19. Also the EPDCCH uses the Demodulation Reference Signals (DM-RS) in contrast to PDCCH. Similar to PDCCH, the EPDCCH is mapped to Enhanced Control Channel Elements (ECCE)s, where each one consists of multiple Enhanced Resource Element Group (EREG)s. In the case of normal cyclic prefix (CP) there are four EREGs per ECCEs, each one consisting in nine resource elements REs. For extend CP and other special cases, there are eight EREGs per ECCEs, each one consisting in eight REs. The number of ECCEs aggregated can be 1, 2, 4, 8, 16 and 32 and their transmission can be localized or distributed with the mapping of ECCEs to EREGs change for each case. As in PDCCH, the EPDCCH also needs to blind decode the ECCEs but there is only specified the

device-specified search space. Also it only supports QPSK modulation [5, 10, 16]. 2.5.4.1.7 Physical Multicast Channel

The PMCH carries the Multicast Channel (MCH) transport channel. This transport channel tranports control and data for Multimedia Broadcast Multicast Service (MBMS) and it is very similar to DL-SCH. The MBMS allows to broadcast/multicast the same information to multiple users in a single network and LTE uses a technique referred as Multicast-Broadcast Single Frequency Network (MBSFN) to allow the transmission. The PMCH is transmitted on MBSFN subframes and each subframe has two parts, control region and data transmission. Only supports extend cyclic prefix (CP) to cover the delay spread from multiple base stations. It supports QPSK, 16QAM, 64QAM and 256QAM modulations [4, 5, 10].

2.5.4.2 Downlink Physical Signals

In order to performe channel estimation, LTE uses a set of reference signals also referred as pilot symbols. They are mapped in the resource grid in unique specific positions which depends on the antenna ports chosen and the cell identity. By using known reference signals to provide a channel response at their position is possible to interpolate them to the entire resource grid and equalize the channel effects [16]. In Table 2.8 the antenna ports defined for each reference signal are shown.

The synchronization signals can be detected by all devices (UE), providing the frame timing and the physical cell identity, which is a number between 0 and 503. It is organized in 168 unique cell identity groups each one containing three identities. Thus the physical cell identity N_IDcell can be defined by:

N_IDcell= 3N_ID(1)+ N_ID(2) (2.8) where N_ID(1) is the cell identity group with a range of 0 to 167 and N_ID(2) is the cell identity within a group with a range of 0 to 2 [10].

2.5.4.2.1 Primary Synchronization Signal

The Primary Synchronization Signal (PSS) is transmitted twice per frame and both in FDD and TDD. In FDD is transmitted on subframes 0 and 5, in the last symbol of slot 0 and in TDD is transmitted in the third symbols of subframes 1 and 6 (slots 2 and 12). The PSS is always mapped in the central 72 subcarriers (6 resource blocks (RB)), but only 62 subcarriers carry information. This feature allows any device (UE) detect the signal regardless the downlink bandwidth used. The signal is created in the base station (eNB) using frequency domain Zadoff-Chu sequences and the two PSS signal within the frame are identical. One of the purposes of the PSS is to provide the cell identity within a cell identity group (N_ID(2)). Also when detected, the Secondary Synchronization Signal (SSS) location is known as the relative location between the both signals is always the same [5, 9]. A more complete description of the signal generation can be found in 3GPP standart TS 36.211 [10].

2.5.4.2.2 Secondary Synchronization Signal

The SSS is transmitted before the PSS. In FDD, it is transmitted on subframes 0 and 5, in second last symbol of slot 0, just before the PSS. In TDD, it is transmitted in the last

symbol of subframes 0 and 5, three symbols before PSS. The SSS is mapped as the PSS, in the central 72 subcarriers but the two SSS signals within the frame are different which allows to find the frame timing and boundary. Also it carries the cell identity group (N_ID(1)) [5, 9]. A more complete description of the signal generation can be found in 3GPP standart TS 36.211 [10].

2.5.4.2.3 Cell-Specific Reference Signal

The CRS is transmitted in all downlink subframes and uses up to four antenna ports (0 to 3) and is only available for a subcarrier spacing of 15 kHz. It is used by devices (UE) to perform channel estimation and for coherent detection of almost all downlink physical channels except PMCH, EPDCCH and PDSCH in transmission modes 7 to 10. In addition, it can also report the Channel State Information (CSI). The CRS is mapped in the resource grid every six subcarriers and every two symbols per slot. The resource elements (RE) used are usually referred as Rp, where p indicates the antenna port used. Also the REs used for

one antenna port are not used for any other antenna port. This can be seen in Figure 2.20, where is illustrated the resource elements mapped for four antenna ports (R0, R1, R2, R3) and

the resource elements unused [5, 10, 16]. A more complete description of the signal generation can be found in 3GPP standart TS 36.211 [10].

2.5.4.2.4 UE-Specific Reference Signal

The UE-Specific Reference Signal also referred as DM-RS is used to coherent demodulation of the PDSCH, by being transmitted on the same resource blocks (RBs) as PDSCH. Therefore the DM-RS is precoded in the same way of PDSCH which allows the UE to directly decode the data. Note that, this signal is intended to a specific user and is only mapped to resources allocated to that user [4, 5, 10]. The DM-RS is transmitted on antenna ports 5, 7, 8, 9, 10, 11, 12, 13, 14, which enable support up to eight DM-RS. In Figure 2.21 a DM-RS mapping for antenna ports 7, 8, 9 and 10 is represented. A more complete description of the signal generation can be found in 3GPP standart TS 36.211 [10].

2.5.4.2.5 Demodulation Reference Signal associated with EPDCCH

The DM-RS associated with EPDCCH is very similar to the UE-Specific Reference Signal but only support up to four DM-RS. Moreover, the DM-RS for PDSCH and EPDCCH are configured independently. The transmission is configured to antenna ports 107 to 110 [5, 10]. A more complete description of the signal generation can be found in 3GPP standart TS 36.211 [10].

2.5.4.2.6 Channel State Information Reference Signal

To support eight layers, the Channel State Information Reference Signal (CSI-RS) was in-troduced and is used to provide the CSI. The CRS already provides this information, however presents some disadvantages. It cannot support up to eight layers and is transmitted every subframe to perform channel estimation for demodulation and provide CSI even when there is not any data. With the introduction of CSI-RS is possible to divide the acquiring of the CSI of the channel estimation increasing efficiency. Also, can be scheduled with different pe-riodicity. The CSI-RS have different configurations, changing the number of reference signals

Figure 2.21: Mapping of downlink UE-Specific Reference Signal for antenna ports 7, 8, 9 and 10 (normal CP) [10]

used. Figure 2.22 illustrates the possible positions for CSI-RS. It can be transmitted on up to eight antenna ports, using the antenna ports 15 to 22 [4, 5]. A more detailed description of the signal generation can be found in 3GPP standart TS 36.211 [10].

Figure 2.22: Possible CSI-RS positions

2.5.4.2.7 Positioning Reference Signal

The Positioning Reference Signal (PRS) allows to determine the location of a device. To calculate the position, the device (UE) use the PRS to measure multiple neighbor base stations (eNB) to calculate the Observed Time Difference of Arrival (OTDOA). Then the network can triangulate the device location. It is transmitted only in downlink subframes configured for PRS and is mapped in diagonal patterns as illustrated in Figure 2.23 and using the antenna port 6 [4, 5, 16]. Further details of the signal generation can be found in 3GPP standart TS 36.211 [10].

2.5.4.2.8 MBSFN Reference Signal

The MBSFN reference signals are transmitted in MBSFN subframes and only when PMCH is transmitted providing channel estimation and coherent demodulation. It uses the antenna port 4. As the MBSFN is defined for extended cyclic prefix (CP) due to large delay spreads, this reference signal is also defined only in the case of extended CP. Multiple base stations (eNBs) transmitts the same MBSFN reference signals pattern in the same time-frequency resources [4, 10, 16]. Figure 2.24 represents the mapping on antenna port4. A more complete description of the signal generation can be found in 3GPP standart TS 36.211 [10].

2.5.4.3 Uplink Physical Channels

As in downlink, uplink also have a Uplink L1/L2 Control Signaling, which carries control information such as hybrid-ARQ, CSI and scheduling requests. The control information is always transmitted, regardless if there is any data to be transmitted in Physical Uplink Shared Channel (PUSCH). There are two methods to transmite the control information:

Figure 2.23: Mapping of PRS [4]