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Understanding and Development of Inter-cell Interference Mitigation
mechanism in LTE-A
Heterogeneous Network
Förståelse och utveckling av Inter-interferens Mitigation mekanism i LTE-A heterogent nätverk
Bilal Shah
Suman Ghimire
Faculty of Health, Science and Technology Master’s Program in Electrical Engineering Degree Project of 30 credit points
Supervisor: Arild Moldsvor (Karlstad University), Pechetty V. Prasad and Bengt Hallinger (Tieto Sweden AB) Examiner: Jorge Solis (Karlstad University)
Date: 5th september 2013 Serial number:
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Abstract
In long term evolution Advanced (LTE-A), concept of heterogeneous network (HetNet) has been introduced. Since, spectrum has become a rare resource these days; another mean is to be looked after to improve the existing wireless technology. One possible way is to improve the network topology so that frequency spectrum can be reused. In heterogeneous network, lower power nodes like Pico/Femto cell are deployed inside Macro cell to increase the system throughput and network coverage. Traditionally, cell selection for user equipment (UE) in LTE is based upon the received downlink power but these Pico/Femto cell have a low power than the Macro cell, meaning that few users can get access from Femto/Pico cells. UE should be close to the Pico/Femto cell to get connected with it. So its solution is; cell selection based upon the uplink path loss can be applied allowing more UE get connected to Pico/Femto cell. On doing that area of the Pico/Femto cell will increase which is called range extension region.
Another problem arises when cell extension is applied, is that Macro cell imposes interference towards the Physical channel and signal of the Pico/Femto cell UE in range extension region as both Macro and Pico/Femto cell operate with same set of frequencies. 3rd Generation Partnership Project (3GPP) LTE-A Enhanced Inter-Cell Interference Coordination (eICIC) scheme has proposed Almost Blank Sub-frame (ABS) as a solution towards the above mentioned interference problem by reducing the activity or muting the Macro sub-frame. So that the corresponding Pico/Femto sub-frame can transmit the user information without interference from Macro cell ABS. For the reason of backward compatibility ABS still transmit certain physical channel and signals like CRSs, PCH, PBCH and PSS/SSS. So, interference still remains in these signals and channels.
The main focus of thesis is reducing the impact of collision of cell-specific reference signal (CRS) from Macro and Femto cell as CRS is used for channel estimation. We have developed LTE link level system model for Macro and Femto cell in the Matlab simulator. Effect of difference in power of Macro and Femto CRS on UE under different noise power is investigated. It shows, higher the power of Macro, higher is the interference level. As a result Femto channel estimation quality degrades which in-turn degrades system performance.
Combined receiver Interference cancelation (IC) methodology is implemented to reduce the impact of interference between macro and Femto CRS collision, which is based upon the reference signal received power (RSRP). System performance is evaluated with bit error rate (BER) and block error rate (BLER) versus Signal to Noise Ratio (SNR) and compared with the single cell system (without interference) and without IC system. Result confirms that IC method system performance is far better than system without IC and as close to system performance of single cell without interference. Furthermore, use of convolutional encoder, offer approximately 7dB coding gain in terms of SNR.
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Acknowledgments
Firstly, we would like to thank Tieto Sweden AB, Karlstad for giving us opportunity to work upon this thesis and providing such a great environment at their office. More importantly, thanks to our company supervisors Pechetty V Prasad, Bengt Hallinger and internal supervisor Arild Moldsvor, We are also thankful to Ireddy Chandra, Shilpika kappa and the whole base band team who helped and guide us in our difficult time.
Secondly, we want to thanks to our families and friends for their continuous support and patience.
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Contents
1 Introduction ... 1
Introduction ... 1
1.1 Goal ... 1
1.2 Problem definition ... 2
1.3 Methodology ... 2
1.4 Previous Work ... 2
1.5 Delimitation and choices ... 3
1.6 Report Outline ... 4
1.7
2 Overview of LTE ... 5
Background ... 5
2.1 2.1.1 Overview of LTE downlink physical layer ... 5
2.1.2 Requirements of LTE ... 5
2.1.3 Multiple Access Method in LTE ... 6
2.1.4 Cyclic-Prefix insertion ... 6
2.1.5 Spectrum flexibility in LTE ... 8
2.1.6 LTE Generic Frame Structure... 9
LTE downlink channel and signals ... 11
2.2 2.2.1 Downlink Reference Signals ... 11
2.2.2 Downlink Physical Channels ... 12
LTE-A Release 10 ... 13
2.3
3 Heterogeneous network ... 15
Introduction to heterogeneous network ... 15
3.1 3.1.1 Advantages of deployment of HeNBs ... 16
3.1.2 Properties of Macro eNBs and LPN ... 16
UE measurements in LTE... 16
3.2 Introduction to range extension ... 17
3.3 Problem caused by deployment of Femto eNBs ... 17
3.4 3.4.1 Evolution of eICIC ... 18
3.4.2 Carrier aggregation in LTE ... 18
3.4.3 Almost Blank Sub-frame ... 19
Colliding and Non colliding CRS ... 20
3.5
4 Channel Model ... 22
Introduction to Air Interface ... 22
4.1 4.1.1 Rayleigh distributed channel ... 23
4.1.2 Rician distributed channel ... 24
4.1.3 Maximum Doppler shift ... 24
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Jake’s Channel model ... 24
4.2
5 Channel Estimation... 26
Channel estimation ... 26
5.1 Signal Model ... 26
5.2 SINR ... 28
5.3 Least Square Estimator ... 28
5.4 Least Minimum Mean Square Error (LMMSE) Estimator ... 29
5.5 Interpolation ... 33
5.6 5.6.1 Frequency Interpolation ... 33
5.6.2 Time Interpolation ... 33
Equalization ... 34
5.7
6 System Model... 36
Introduction to System Model ... 36
6.1 Encoder ... 36
6.2 6.2.1 Cyclic redundancy check (CRC) ... 37
6.2.2 Convolutional encoder ... 37
Digital modulation ... 39
6.3 6.3.1 QPSK ... 39
6.3.2 QAM ... 39
MIMO- STBC ... 40
6.4 Mapping of signal to the grid ... 40
6.5 DFT/IDFT ... 40
6.6 Cyclic Prefix (CP) ... 42
6.7 Power Amplifier ... 42
6.8 Air Interface ... 44
6.9 Receiver ... 44
6.10 Core methodology ... 45
6.11 6.11.1 Direct IC ... 47
6.11.2 Joint channel Detection ... 48
6.11.3 No IC ... 50
7 Simulation and Results ... 51
Simulation set up ... 51
7.1 7.1.1 NO IC Scheme Based results ... 52
7.1.2 Conclusion based on No IC scheme ... 56
7.1.3 IC Scheme based Results ... 56
7.1.4 Conclusion based on IC scheme ... 59
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8 Conclusion and Future work ... 60
Conclusion ... 60
8.1 Future Work ... 61
8.2
9 References ... 62
10 Appendix A ... 65
A.1 LTE resource grid for LTE minimum bandwidth (1.4 MHz) ... 65
A.2 LTE resource grid for maximum bandwidth (20 MHz) ... 66
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Table of figures
Figure 2.1: OFDM Orthogonal Sub-carriers ... 6
Figure 2.2: Simple OFDM System ... 6
Figure 2.3: Graphical representation of Cyclic Prefix (CP) ... 7
Figure 2.4: OFDM and OFDMA graphical concept ... 8
Figure 2.5: FDD and TDD ... 8
Figure 2.6: LTE time domain frame structure. ... 9
Figure 2.7: LTE time frequency resource grid ... 10
Figure 2.8: LTE CRSs Position in a single Antenna ... 11
Figure 2.9: LTE CRSs mapping for 2x2MIMO ... 12
Figure 3.1: Heterogeneous Networks ... 15
Figure 3.2: Range extension in Heterogeneous networks ... 17
Figure 3.3: Illustration of CSG and range extension user ... 18
Figure 3.4: Illustration of Carrier aggregation in LTE ... 19
Figure 3.5: Illustration of ABS in LTE ... 20
Figure 3.6: Illustration of Colliding and Non Colliding CRS ... 21
Figure 4.1: Multipath effect [25] ... 22
Figure 5.1: LTE time domain and frequency domain equalizer option ... 34
Figure 6.1: System model ... 36
Figure 6.2: Code rate 1/3, Convolutional encoder ... 38
Figure 6.3: Convolutional encoding gain ... 38
Figure 6.4: QPSK state diagram ... 39
Figure 6.5: QAM scheme diagram. A) QAM-16 B) QAM-64 ... 39
Figure 6.6: DFT Block diagram ... 41
Figure 6.7: IDFT Block diagram ... 41
Figure 6.8: AM/AM & AM/PM mapping ... 42
Figure 6.9: RAPP Amplifier smoothness factor is varying ... 43
Figure 6.10: RAPP Amplifier, Saturation level varying ... 43
Figure 6.11: 2x2 MIMO Configuration over air interface ... 45
Figure 6.12: Flow chart of IC Algorithm ... 46
Figure 7.1: CSR, ABS and PDSCH configuration in LTE ... 52
Figure 7.2: UE receiving signal from Femto and Macro eNB ... 52
Figure 7.3: BER when Femto RSRP is varying for QPSK ... 53
Figure 7.4: BER when Femto RSRP is varying for QAM-16 ... 53
Figure 7.5: BER when Macro RSRP is varying for QPSK ... 54
Figure 7.6: BER when Macro RSRP is varying for QAM-16 ... 54
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Figure 7.7: BER when SNR is varying for QPSK ... 55
Figure 7.8: BER when SNR is varying for QAM-16 ... 56
Figure 7.9: BER when SNR is varying for QPSK using IC scheme ... 57
Figure 7.10: BER when SNR is varying for QAM using IC scheme... 58
Figure 7.11: Block error rate for QPSK, 1500 sub-frames ... 58
Figure 7.12: Block error rate for QAM-16, 1500 Sub-frames ... 59
Figure A.1: LTE Resource grid for 1.4MHz bandwidth ... 65
Figure A.2: LTE Grid for 20MHz bandwidth ... 70
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List of Tables
Table 2.1: LTE downlink Bandwidths ... 9
Table 2.2: IMT advance requirements ... 13
Table 2.3: A comparisonof LTE and LTE-A Features and Characteristics [15] ... 14
Table 5.1: value for different modulation scheme and Code rate [35] ... 32
Table 7.1: Parameter used in Simulation ... 51
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List of Abbreviations
3G 3rd Generations
3GPP 3rd Generation Partnership Project
4G 4th Generations
A
ABS Almost Blank Sub-Frame
AWGN Additive White Gaussian Noise
B
BER Bit Error Rate
BLER Block Error Rate
C
CA Carrier Aggregation
CC Component Carrier
CCPCH Common Control Physical Channel
CDMA Code Division Multiple Access
CFR Channel Frequency Response
CIR Channel Impulse Response
COMIC Combined Interference Cancellation
CP Cyclic Prefix
CRC Cyclic Redundancy Check
CRS Cell Specific Reference Signal
CSG Closed Subscriber Group
D
DDCE Decision Directed Channel Estimation.
DFT Discrete Fourier Transform
DPSK Differential Phase-Shift Keying
DVB-S Digital Video Broadcasting-Satellite
E
EDGE Enhanced Data Rates for GSM Evolution
eICIC Enhanced Inter-Cell Interference Coordination
eNB Evolved Node Base Station
E-UTRA Evolved Universal Terrestrial Radio Access
ER Extension Region
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F
FDD Frequency-Division Duplex
G
GSM Global System for Mobile Communication
H
HeNB Home Evolved Node Base Station
HetNet Heterogeneous networks
HPN High Power Node
HSDPA High-Speed Downlink Packet Access
I
IC+DDCE IC-Assisted Decision Directed Channel Estimation
ICIC Inter-Cell Interference Coordination
ID Identification
IDFT Inverse Discrete Fourier Transform
IFFT Inverse Fast Fourier Transform
IMT International Mobile Telecommunication
ISI Inter-Symbol Interference
ITU International Telecommunication Union
L
LMMSE Least Minimum Mean Square
LOS Line Of Sight
LPN Low power networks
LS Least Square
LTE Long Term Evolution
LTE-A Long Term Evolution advanced
M
MIMO Multiple Input Multiple Output
MMOG Multimedia Online Gaming
O
OFDM Orthogonal Frequency Division Multiplex
OFDMA Orthogonal Frequency Division Multiple Access
P
PBCH Physical Broadcast Channel
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PCC Primary Component Carriers
PCH Paging Channel
PDCCH Physical Downlink Control Channel
PDF Power Delay Profile
PDSCH Physical Downlink Shared Channel
PHY Physical Layer
PRB Physical Resource Block
PSK Phase-Shift Keying
PSS Primary Synchronization Signals
Q
QAM Quadrature Amplitude Modulation
QPSK Offset Quadrature Phase Shift Keying
R
RB Resource Block
RSRP Reference Signal Received Power
RSRQ Reference Signal Received Quality
RSSI Received Signal Strength Indicator
RX Receiver
S
SCC Secondary Component Carriers
SNR Signal to Noise Ratio
SSPA Solid State Power Amplifier
SSS Secondary Synchronization Signals
SIB System Information Block
T
TDD Time-Division Duplex
TWTA Traveling Wave Tube Amplifiers
TX Transmitter
U
UE User Equipment
UMTS Universal Mobile Telecommunications System
W
Wi-Fi Wireless Fidelity
WiMAX Worldwide Interoperability for Microwave Access
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Chapter 1
Introduction
Introduction 1.1
LTE is developed by the 3rd Generation Partnership Project (3GPP) in order to make sure effectiveness of its standards in long term. Recently it is also known as 4th generation technology. LTE is the evolution of 3rd generation mobile technology also called as Universal Mobile Telecommunications System (UMTS). The main challenges for LTE is to come up with new radio access technology so that high data rates, low latency can be offered. LTE is also known as E-UTRAN (Evolved UMTS Terrestrial Radio Access Networks).
Orthogonal frequency division multiplexing (OFDM) is the basis technology for the long term evolution (LTE) where the channel estimation is done with the help of pilots or in LTE term reference signals. LTE-A supports, heterogeneous network deployment which eliminate the coverage holes of Macro base station and offers higher data rate and capacity. However such deployments give rise to inter cell interference. In LTE-A, when inter-cell interference occurs in HetNet Base station with higher power transmits ABS (almost blank sub-frame) to reduce the interference however still there is a possibility of interferences due to the collision of reference signal. General receivers in this situation cannot accurately estimate the channel so advanced receiver with interference cancellation scheme is required for UE.
This master thesis investigates on the inter-cell interference caused by the collision of reference signal from Macro and Femto eNBs and we have developed an interference mitigation mechanism based upon the RSRP. It follows the specification of 3rd Generation Partnership Project (3GPP) LTE-A. Detail description of LTE and OFDM is presented in chapter 2. This chapter includes the Goal, Problem definition, Methodology, Previous work and Thesis outline.
Goal 1.2
In General, goal of this thesis can be summarized in the following points.
To understand overall LTE system
To understand the interference that exists in heterogeneous networks
*To investigate on the 3GPP LTE interference cancellation and coordination schemes like ICIC and eICIC
**Investigation on mechanism for successful decoding of the control and traffic information under interference condition
*Modeling & simulation of physical layer for the Evolved Node Base Station (eNB), Home Evolved Node Base Station (HeNB), UE and Channel conditions in Matlab
**Verifying the model with different power level of eNB and HeNB
Evaluating and find out BER/BLER at different SNR to check the system performance
(Note: Throughout the thesis we have worked together, however there are some task that is contributed mostly by individual. In the above ‘*’ represents the major contribution made by Bilal Shah and ‘**’ represents the major contribution made by Suman Ghimire)
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Problem definition
1.3
The purpose of the project is to understand the interference between the Macro and Femto cells with respect to the CRSs of Macro and Femto cell in heterogeneous Network which operate with same set of frequencies, and come-up with mechanisms to minimize the inter cell interference.
LTE Release 10 has the feature called eICIC which is developed to mitigate the interference due to collision of CRSs from Macro and Femto cell. The eICIC is able to remove the interference towards the data signal of the corresponding sub-frames of the co-channel victim Femto cell with the help of Almost Blank Sub-frame (ABS). However, even with the usages of ABS, interference from CRSs and other physical signals still appears. For the reason of backward compatibility ABS transmits certain physical channel and signals like CRSs, PCH, PBCH and PSS/SSS. In this work, we have considered the interference due to the collision of CRSs from Macro and Femto cell. As CRSs are primarily used for the channel estimation and collision of CRSs can significantly reduce the system performance. So, we have investigated and developed a mechanism to minimize this interference caused by the collision of CRSs of Macro and Femto cell.
The concept of colliding and non-colliding case of CRSs interference is described more specifically and in depth detail in the section 3.5.
Methodology 1.4
LTE employs Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA) technology as the multiplexing scheme for Downlink and uplink respectively.
We have considered the Heterogeneous network where low power base stations are deployed inside High power base stations. We have modeled Macro Base station as a high power node (HPN) and Femto base station as low power node (LPN). Moreover, UE is in the cell edge region which also called cell extension region in LTE term, trying to access Femto cell. At the same time UE is subject to high interference from Macro cell. Cell ID uniquely represent the cell and in our thesis it is used for CRS generation. RSRP is used in channel estimation and also for the measurement of system performance.
The IC algorithm is based on the received signal received power (RSRP) of both Macro and Femto cell as cell ID and RSRP of both cells are known to UE. When UE receives a combine signal from Macro and Femto Base Station, Based upon the RSRP difference of Macro and Femto cell UE Estimate the Macro signal or Interfering signal with direct IC method or Joint detection method and subtract it from the total combined signal to get the desired reference signal from the Femto cell. Which is further used in channel estimation of Femto reference signal and then interpolation is done to get the channel estimation of data signal. Detail description of System Architecture, there choices and Limitation and a mathematical and theoretical approach to minimize the interference is presented in Chapter System model.
Previous Work 1.5
Since this interference problem is lately discovered and still the hot topic among LTE professionals, only few works have been done in Similar Interference scenario which can be found in the [1], [2], [3] and [4]. The Interference cancellation technique used in this work is modification and extension of [2] and [3].
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In [1], Interference is due to the collision between reference signal and synchronization signals, In this paper methodology is not described as Receivers algorithm are designed by Manufacturers and they don’t usually disclose the methodology.
In [2], there is one macro cell, one Femto cell and one cell edge user in a Heterogeneous Network. The interference is due to the collision of reference signal from both Macro and Femto cell. Combined IC, Decision direct Channel Estimation and IC assisted Decision direct Channel Estimation has been proposed. Results show that proposed methodology significantly reduces the interference. In [3] the methodology is same but they have modeled more than one Interfering cell i.e. Macro cell.
In [4], 3GPP ICIC technique is used where first, it is decided how much power Macro BS can transmit so that Pico ER UEs get the desired DL SINR. This requires some form of coordination between macro BSs and a Pico BSs. On the basis of decided power Macro BS allocate RBs and transmit powers to its DL UEs, while respecting the power constraints previously derived to support Pico ER UEs.
In the above mentioned references all other except [4] have used time domain eICIC method for interference minimization. Beside above mentioned technique, sub-frame shift is a solution to the inter-cell interference problem but sub-frame shift cannot be applied to LTE TDD system and another method could be configuring ABS to be MBSFN, However MBSFN cannot be always configurable.
Delimitation and choices 1.6
This project work is mainly focused upon the LTE Downlink so we have implemented OFDMA system and we have followed the com-IC method of [2], we have used 20 MHz bandwidth, Jakes Channel Model, Convolutional encoder, STBC, QPSK, QAM and RAPP power amplifier. We have measured the system performance with BER and BLER, However In [2], 5 MHz bandwidth, QPSK, WINNER II C2 (=EVA) and C3 (= ETU) channel model and Turbo coding is used. The system performance is measured with System throughput and BER. In [3], they have followed the same approach with theoretical information but considered more than one Macro Interfering Cell.
The reason for the different choices we used is generally for easy implementation to avoid complexity and more specifically to improve system performance. We implemented CRC and convolutional encoder which are for error detection and correction respectively and also to check the packet error rate or block error rate of the system which is more effective way to analyses the system performance. The implementation of STBC delivers transmits antenna diversity. Alamouti Scheme is one of the simplest schemes of STBC and more specific for 2x2 MIMO configurations and has full rate code i.e. rate 1code. In modulation scheme QPSK is simplest for low and reliable data rate communication whereas QAM is demanded for the high data rate in digital communication. Macro and Femto cell which are commonly differentiated from each other in terms of power and user, so RAPP power amplifier fulfill our requirements of power variation of transmit signal and it is simple and easy to implement.
Modeling of channel is complex and time consuming. So, we have used Jake’s channel model provided by Company in order to save time.
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Report Outline
1.7
The outline of this master thesis is as follows:
Chapter 2: Describes the detail of LTE downlink physical layer along with OFDM and, reference signals.
Chapter 3: Describes the theory of heterogeneous network along with UE measurements in LTE, range extension and eICIC
Chapter 4: Describes the air Interface which includes Jakes Channel model.
Chapter 5: Describes the theory behind Channel estimation technique like Least Square (LS) error and Least Minimum Mean Square Error (LMMSE) and Interpolation that is used in the methodology.
Chapter 6: Demonstrate the overall system model and present important aspect of the model.
Chapter 7: Analyses the simulation, results and finding of our thesis.
Chapter 8: Describe the conclusion that is drawn from the work and possible extension of the work in the future.
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Chapter 2
Overview of LTE
Background 2.1
The recent increase of mobile data usage and emergence of new applications such as MMOG, mobile TV, Web 2.0, streaming contents have motivated the Third-generation Partnership Project (3GPP) to work on the Long term evolution (LTE). The main objectives of LTE are to minimize the system and User Equipment (UE) complexities, allow flexible spectrum deployment in existing or new frequency spectrum and to enable co-existence with other 3GPP Radio Access [5]
The third Generation Partnership Project (3GPP) has introduced LTE as a next generation IP- based OFDMA technology. The reason is to facilitate increasing demand of mobile data usage and new multimedia applications. LTE is advancement of GSM/EDGE and UMTS/HSDPA network technologies with additional capability to support bandwidth wider than [6].
2.1.1 Overview of LTE downlink physical layer
LTE physical layer is designed by considering the requirements for high data rate, spectral efficiency, and multiple channel bandwidths. These requirements can be fulfilled by using orthogonal frequency division multiplex (OFDM).
OFDM is a technology that was published in 1960’s. In the 1980’s, OFDM has been studied for high speed modems. It was considered for 3G systems in the mid-1990s before being determined too immature. Developments in electronics and signal processing since that time has made OFDM a mature technology widely used in other access systems like 802.11 (Wi- Fi) and 802.16 (WiMAX) and broadcast [7].
In addition to OFDM, MIMO (multiple input multiple output) is another basis technology that helps meeting the requirement of an LTE. It can either increase channel capacity (spatial multiplexing) or enhance signal robustness (space frequency/time coding).
2.1.2 Requirements of LTE
Higher User throughput of 3-4 times that of HSDPA (Rel.6) in downlink and 3-4 times that of enhanced Uplink (Rel.6).
Mobility optimized for low mobile speed from 0 to , also supports higher mobile speeds.
Optimal cell size of 5 km, 30 km sizes with reasonable performance and up to 100 km cell sizes supported with acceptable performance.
Supports a large number of users per cell of at least 200 users/cell
Spectrum flexibility with 1.4, 3, 5, 10 ,15 and Bandwidth
Co-existence with legacy standards
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2.1.3 Multiple Access Method in LTE
OFDM is a multicarrier transport technology for high data rate communication system. The main concept in OFDM is spreading high speed data over the low rate carriers. These low rate carriers are called sub-carriers. Sub-carriers are generated using IFFT (Inverse Fast Fourier Transform) digital signal processing. IFFT is an efficient scheme for generating orthogonal subcarriers. These orthogonal subcarriers are mutually orthogonal in frequency domain which avoids the inter-symbol interference (ISI) as shown in figure 2.1 below. Each sub-carrier has a bandwidth less than channel coherence bandwidth which is why these subcarriers experience flat fading.
f
Figure 2.1: OFDM Orthogonal Sub-carriers
Firstly, Bit stream from the encoder are modulated using QPSK or QAM-16 or QAM-64 into symbols. These symbols are then fed to serial to parallel converter. The parallel data symbols from serial to parallel converter are then spread over N IFFT orthogonal subcarriers. The meaning of orthogonal subcarrier is that there is no interference between the subcarriers.
Again the outputs from the IFFT which are modulated subcarriers are converted back to the serial form with the help of Parallel to serial converter. This again passed through the digital to analog converter before transmitting to the air as shown in figure 2.2. In the receiver the reverse operation is done to the OFDM symbol to retrieve the data stream.
Serial to IFFT parallel
Parallel to Serial
Digital To Analoge Convertor Modulato
Bit stream r from encoder
N complex Symbols
Figure 2.2: Simple OFDM System
2.1.4 Cyclic-Prefix insertion
One of the drawbacks of OFDM signal (Block of transmitted carriers) is that when it is propagating through the channel i.e. free air, it convolves with channel impulse response in
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time domain, and after convolving with channel the length of convoluted signal increase by factor of (L-1), where L is the length of channel impulse response. Due to this increase of length (L-1), symbols will overlap with next consecutive symbol and is called ISI. Reason for adding CP is illustrated graphically in the figure 2.3.
X[2]
X[0] X[1] . . . X[M-1] X[M]
l+1 symbol X[2] . . . X[M-1] X[M]
X[0] X[1]
l symbol
X[M-1]
. . . X[M]
Symbol Over lap (L-1)
l symbol[M+L-1]
X[M-1]
. . . X[M]
Symbol Over lap (L-1)
Symbol Over lap (L-1) l+1 symbol[M+L-1]
A
B
X[M-1]
. . . X[M]
. . . X[M]
X[M-L+1] X[0]
l symbol
X[M-1]
. . . X[M]
. . . X[M]
X[M-L+1] X[0] X[M-L+1] . . . X[M]
C
CP (L-1) CP (L-1) CP (L-1)
Discard CP X[n] C[n] Discard CP X[n] C[n] Discard
D
Figure 2.3: Graphical representation of Cyclic Prefix (CP)
In above figure 2.3(A), any two transmitted symbol I and I+1 of length M are considered, whereas in figure 2.3(B), the channel impulse response (CIR) of length L is considered when it convolved with two symbols its length become M+L-1 instead of M. So the length L-1 of symbol I overlap the next I+1 symbol and over write Data there. To overcome this difficulties CP is appended to the front OFDM symbol and which is the copy of last (L-1) subcarriers of that symbol. After insertion of CP to all OFDM symbol and then convolving with channel impulse response the received symbol get affected only in the CP part as shown in figure 2.3(C). This way actual symbol can be kept safe and reduce the ISI. After receiving the symbol we discard the CP part in the receiver as shown in figure 2.3(D).
The disadvantage of CP insertion is that it does not carry any new data information, which decreases the transmitted energy per information bit, so there is power loss and reduced the SNR [8], second there is reduction in symbol rate which is the loss in terms of bandwidth and can be reduced by decreasing the inter carrier spacing [9], [10].
In LTE physical layer, Orthogonal Frequency Division Multiple Access (OFDMA) scheme is used for downlink transmission. OFDMA make use of OFDM technology to multiplex different users by assigning specific patterns of subcarriers in time and frequency domain. The scheduling and assignment of resources makes OFDMA different from OFDM. In the OFDM figure 2.4 shows that the entire bandwidth is assigned to single user for a certain period of time but in OFDMA multiple users are sharing the bandwidth at each point in time [11].
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Sub Carriers
Time Time
OFDM OFDMA
Sub Carriers
Figure 2.4: OFDM and OFDMA graphical concept
2.1.5 Spectrum flexibility in LTE
One of the main characteristics of LTE radio-access technology is Spectrum flexibility which allows deployment of LTE radio-access in different frequency bands with different sizes since spectrum has become a scarce resource. This flexibility includes 2 main areas as follows:
Flexibility in duplex arrangements
Bandwidth flexibility
In Flexibility in duplex arrangements, the communication can take place both in paired and unpaired bands. In former case the uplink and downlink transmissions use separate frequency bands, while in later case downlink and uplink share the same frequency band. LTE support both Frequency-Division Duplex (FDD) and Time-Division Duplex (TDD).
In FDD uplink and downlink transmissions take place in different and sufficiently separated frequency bands, whereas in TDD uplink and downlink operate in different non-overlapping time slots as shown in figure 2.5. In our thesis we have considered only FDD.
Time (t) Time (t)
Frequency (f) Frequency (f)
F-DL
F-UL
F-DL+ F-UL
FDD TDD
Figure 2.5: FDD and TDD
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Bandwidth flexibility is an important aspect in the LTE operation. It is due to the fact that LTE can be operated in different transmission bandwidths for uplink and downlink. The reason for having different bandwidth is that LTE deployment depends upon the frequency bands and on the operator. This bandwidth flexibility arise chances of gradual frequency bands migration from other radio-access technologies.
2.1.6 LTE Generic Frame Structure
OFDMA is the best radio access for LTE downlink comparing to others. However, resource allocation is complicated in OFDMA. It is far better than packet oriented approaches in the context of efficiency and latency. In LTE, users are assigned a certain number of subcarriers for predetermined period of time; these subcarriers are called physical resource blocks (PRBs). These blocks have both time and frequency dimension [12].
Figure 2.6: LTE time domain frame structure.
As can be seen in the figure 2.6, LTE frame is of 10 millisecond ( ) duration which is divided into 10 sub-frames of 1msec duration. Each sub-frame is further divided into two slots each has a length of 0.5 . Each slot comprises either 6 or 7 OFDM symbols depending upon normal or extended CP is used.
Table 2.1: LTE downlink Bandwidths
Bandwidth (MHz) 1.4 3 5.0 10.0 15.0 20.0
Subcarrier bandwidth (kHz)
15
Physical resource block (PRB)
bandwidth (kHz) 180
Number of available PRBs
6 12 25 50 75 100
10
Table 2.1 shows LTE specification defines different channel bandwidths from 1.4 to 20 . Different channel bandwidth has different number of available PRBs. A PRB is comprises of 12 consecutive subcarriers for one slot as shown in figure 2.7 below. A PRB is the smallest unit that base station can allocate to the user. Subcarrier bandwidth and PRBs bandwidths are 15 and 180 respectively for all system bandwidths.
The figure 2.7 is shown for the case of normal CP. The transmitted downlink signal consists of subcarriers for the duration of OFDM symbols.
1 Frame (10 msec) 1 Sub-Frame (1.0 msec)
1 Slot (0.5 msec)
12 subcarriers
N subcarriers
RESOURCE BLOCK 7 symbols x 12 subcarriers (normal cp), or
6 symbols x 12 subcarriers (extended cp)
Resource element
Time domain symbol
Frequency domain sub-carriers
Figure 2.7: LTE time frequency resource grid
A Resource element is the smallest physical resource and it consists of one subcarrier during one OFDM symbol. A group of resource elements are referred as physical resource blocks i.e.
PRBs. A PRB has a seven OFDM symbols having duration of one time slot 0.5ms and 12 subcarriers having a bandwidth of ( , so each resource block in the case of normal cyclic prefix has of 84 resource elements ( ) whereas in the case of extended cyclic prefix resource block has 72 resource elements ( .
11
LTE downlink channel and signals 2.2
2.2.1 Downlink Reference Signals
Packet oriented networks use PHY preamble to facilitate carrier offset estimate, channel estimation, timing synchronization etc. However, in LTE special reference signals are mapped into the PRBs to allow coherent demodulation at the user equipment. Downlink reference Signals are placed in the first and third last OFDM symbol in each slot. It means for the case of normal CP reference signals are depicted in first and fifth OFDM symbol and of the extended CP reference signals are depicted in first and fourth OFDM symbol. Frequency spacing between two reference signals is six subcarriers. Therefore there are four reference signals in one PRB. These reference signals are used to estimate the channel response on subcarriers bearing them. Interpolation is employed to estimate the channel response on other subcarriers [12]. Figure 2.8 given below illustrated are for single antenna LTE system with normal CP.
0 1 2 3 4
6 7 8 9 10
1 2 3 4 5 6 0 1 2 3 4 5 6
R
R
R
R
R
R
R R Slot 0
Subframe
12 Su bc ar rier s
Slot 1
Figure 2.8: LTE CRSs Position in a single Antenna
In the case of 2x2 MIMO operations, the receiver must determine channel impulse response from each transmitting antenna. For the accurate channel estimation, when one antenna is sending the reference signal other antenna does not transmit anything at the same time and frequency position of reference signals. Mapping of reference signals for 2x2 MIMO operations is shown in figure 2.9.
12
Reference signal From Antenna 0 Reference signal From Antenna 1 Unused Resource
element Subcarriers (frequency)
Antenna 0
OFDM Symbols (time)
Antenna 1
X
X
X
X X
X
X
X
X
X
X
X X
X
X
X
R1
R0
R0
R0
R0 R0
R0
R0 R0
R1
R1
R1 R1
R1 R1
R1
R0
R1
X
Figure 2.9: LTE CRSs mapping for 2x2MIMO
In our thesis we have generated the reference signals as per the technique described in 3GPP TS 36.211 version 10.0.0 Release 10 [13] and mapping to resource grid is done as described above. We have used cell ID 300 for Macro cell and 3 for the Femto cell so that CRS of Macro and Femto collide. CRS collide with each other when criteria given by equation (2.1) is fulfilled otherwise they don’t collide.
( ) ( ) (2.1)
Where, and is Macro and Femto cell ID.
2.2.2 Downlink Physical Channels
There are mainly three different types of physical channels are defined for LTE downlink.
Physical channels are used for conveying information from higher layers to the LTE stack.
However, Physical signals like reference signal and synchronization signal are used just in the physical layer. LTE downlink physical channels are
Physical downlink shared channel (PDSCH)
Physical downlink control channel (PDCCH)
Common control physical channel (CCPCH)
PDSCH is used for transportation of data and multimedia. Modulation schemes for PDSCH include QPSK, QAM-16 and QAM-64 as they are designed for high data rates.
PDCCH is generally used for transportation of UE specific control information. Reliability and robustness are the important aspect for PDCCH so only QPSK is available as a modulation scheme.
CCPCH is used for carrying cell wide control information. As PDCCH, reliability and robustness are the key aspect for CCPCH also, therefore only QPSK is available as a modulation scheme. They are transmitted on the 72 active subcarriers centered on the DC subcarrier.
13
All the down link physical channels and signals and their specific mapping in LTE resource grid for bandwidth 1.4MHz and 20MHz is shown in Appendix B.1 and B.2 respectively.
LTE-A Release 10 2.3
The LTE release 10 was introduced in order to fulfill the IMT (International Mobile Telecommunication) -Advanced requirements; some of these are listed in table 2.2. The ‘‘IMT is a global broadband multimedia international mobile telecommunication system that the ITU (International Mobile Telecommunication) has been coordinating along with governments, industry and private sector. IMT-Advanced is the term that ITU uses to describe radio-access technologies beyondIMT-2000’’ [14].
Table 2.2: IMT advance requirements
Channel bandwidth Supporting up to Peak spectral
efficiencies
15 bit/s/Hz in down link and corresponds to peak rate of 6.75 bit/s/Hz in uplink and corresponds to peak rate of 270Mbits/s Control plane latency less than
User plane latency less than
The LTE release 10, radio-access technology is fully compliant with the IMT-advanced requirements and the main reason for LTE release 10 to be called LTE-Advanced.
The Carrier aggregation, extended multi–antenna transmission, almost blank sub-frame (ABS), relaying and Heterogeneous deployments are some of the important enhancements and features introduced in LTE Advance. Table 2.3 shows the feature and characteristics of LTE and LTE-A for comparison purposes.
14
Table 2.3: A comparison of LTE and LTE-A Features and Characteristics [15]
15
Chapter 3
Heterogeneous network
Introduction to heterogeneous network 3.1
Recently the number of mobile broadband users has been grown exponentially and it will definitely grow more in the future. This is due to the fact that evolution on terminal capabilities which can handle new and different kind of services. It is predicted that traffic per year is going to be double in next five years, therefore by 2014 average traffic uses 1GB of data per month compared to 100 or 200 MB now [16]
Scarcity of frequency spectrum is always a big issue. Spectral efficiency of wireless networks is reaching its theoretical limits, it is necessary to increase the node density which can improve the network capacity. If macro cell deployments are sparse then adding another cell does not create significant inter-cell interference and cell splitting gain can be achieved.
However, if macro cells are already dense, cell splitting gains are significantly reduced because of inter-cell interference. Instead of deploying macro cell another low power nodes can be deployed. These low power nodes are called Pico cell and Femto cell. Those network which consist of macro and low power nodes are called heterogeneous network [17]. In heterogeneous network Pico/Femto cell with same set of frequencies are deployed within the Macro cell in an unplanned way depending upon the areas that has more traffic. In one hand these deployment increases the overall network capacity, data rate and the performance of the cell-edge user and on the other hand such deployments cause severe Macro-Pico/Femto inter- cell interference. Figure 3.1 shows the typical scenario of heterogeneous networks, where in each cell there is one Macro base station, one Femto base station and one user equipment i.e.
UE.
Femto Base Station
Macro Base
Station UE
Figure 3.1: Heterogeneous Networks
In this thesis Femto cell and Pico cell are considered as same. Therefore we have taken Femto cell that represents low power node (LPN). We have also considered the scenario where Femto cell is deployed inside macro cell. In 3GPP LTE term base station is referred as
16
evolved node base station also abbreviated as eNBs for macro base station and HeNBs for Femto base station.
3.1.1 Advantages of deployment of HeNBs
They can be deployed to eliminate coverage holes
Offer high data rate and capacity where they are deployed
less costly and easy to deploy comparing to macro cell
do not require air conditioning unit in the power amplifier
Cell splitting gain can be achieved
Traffic offloading to the low power nodes can be done
3.1.2 Properties of Macro eNBs and LPN
Macro eNBs
Power range around +45dBm
Covers outdoor with around cell site ~5km
Operator deployed
Femto eNBs
Power range around +15dBm
Covers indoor residence 50m
Operator deployed or User deployed
Equipped with Omnidirectional antenna
Backhaul uses the existing internet connection such as DSL
Pico Cell [17]
Power range around 23.97dBm for outdoor and 20dBm
Pico cell are regular macro cell except for low transmitting power
Equipped with Omnidirectional antenna
Backhaul uses the X2 interface for data communication and interference management
Relay [17]
Low power nodes (LPN) used in HetNets to enhance coverage.
No wired Backhaul ,wireless backhaul by relaying the signal from mobile station to Macro eNB
In-band relay node uses the same UL and DL frequency where in out-of-band uses different frequencies in UL and DL.
UE measurements in LTE 3.2
Reference signal received power (RSRP) is defined as the as the linear average over the power contributions ( of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth .For RSRP determination the cell-specific reference signals (from first antenna) according to TS 36.211 [18] be used. If the UE can reliably detect that (from second antenna) is available it may use in addition to to determine RSRP [19].
17
Reference signal received quality (RSRQ) is defined as the ratio N×RSRP / (E-UTRA carrier RSSI), where N is the number of RB’s of the E-UTRA carrier RSSI measurement bandwidth.
The measurements in the numerator and denominator shall be made over the same set of resource blocks. E-UTRA Carrier Received Signal Strength Indicator (RSSI), comprises the linear average of the total received power ( observed only in OFDM symbols containing reference symbols for antenna port ‘0’, in the measurement bandwidth, over N number of resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc. [20].
Introduction to range extension 3.3
Cell selection in LTE is done based upon UE measurements of RSRP. In traditional Macro cell networks the eNB having the high RSRP is selected [21]. However, in a heterogeneous network there are base stations transmitting with different power level. So it is unfair to the base station transmitting with lower power because terminal always chooses higher power base station. This problem can be minimized if cell selection is done based upon uplink path loss, which in practice is done by applying cell specific offset of the RSRP and RSRQ. This method of cell selection increases the coverage area of the low power base station and this increased area is named as range extension as shown in figure 3.2.
Range extension region
Figure 3.2: Range extension in Heterogeneous networks
Problem caused by deployment of Femto eNBs 3.4
As it is mentioned above that Femto eNBs which are also called HeNBs can be operator deployed or user deployed. If it is operator deployed Macro user also have access to it which is called open access but if it is user deployed Macro user cannot get connected to it only limited number of users like family members of that particular home will have access to it which is called closed subscriber group (CSG).
In this situation when Macro UE (user equipment) is in edge of Femto cell there will be strong interference from Femto cell and may even not be able to access the Macro cell at all. In another situation strong Macro cell interferes the Femto UE. This happens when sometime it is preferable to connect UE to the Femto cell even if the received power from the Femto cell is weaker than Macro cell. This is useful when strong cell has weak backhaul quality or when it is necessary to offload the traffic to Femto cell and to achieve true cell-splitting gains in the network [22].
Let’s take an example when UE is in cell extension region and it is connected to Femto base station. But received power by the UE from the Macro is much higher than the power received
18
from Femto. This Power difference cause severe interference to the Femto user by Macro base station. This scenario can be demonstrated by the figure 3.3.
Range extension region UE1
UE2
Figure 3.3: Illustration of CSG and range extension user
As we can see UE2 is in well inside the Femto cell which is also called center Femto user.
Here, the received power of the Femto is stronger than the received power of Macro so there is no issue of interference. But if we see the UE2 it is in the cell extension region and it is connected to the Femto base station. Received Power by this user from the Macro is much higher than power received from Femto. So, UE2 undergoes severe interference imposes by Macro base station.
So this is one of the main disadvantages of heterogeneous network. Our thesis deals with the minimization of this interference. We have assumed the heterogeneous network of Macro and Femto base station. And there is an UE in the cell extension region which is interfered by the Macro base station.
3.4.1 Evolution of eICIC
In order to reduce Inter-cell interference in the LTE, the Release 8 of the 3GGP standardization body proposed the ICIC (Inter-cell Interference Coordination) feature. It is one of the new RRM (Radio Resource management) functions. The task is to determine the resources (frequency time and power) available in each cell and schedule those resources to the users. The ICIC, as described in [23], is located in the eNB and it has the task of managing the PRBs such that inter-cell interference is kept under control. The coordination between neighboring eNBs is based on the exchange of interference information. The main problem with ICIC is that it is able to mitigate the interference between data channels but control channels are still under interference.
In LTE-A enhanced Inter-Cell Interference Coordination (eICIC) is introduced to address this problem. They proposed two methods one is frequency domain multiplexing inter-cell interference coordination scheme and other is Time domain multiplexing inter-cell interference coordination scheme (Almost Blank Sub-frame).
3.4.2 Carrier aggregation in LTE
Carrier aggregation is used in Frequency domain multiplexing inter-cell interference coordination scheme which is one of the most important features of LTE-A. This scheme allows LTE-A user to connect to several carriers at the same time. It also allows resource
19
allocation across the carriers as well as fast switching between carriers without time consuming handovers, which means a node, can schedule its control and data information on separate carriers. Main principle in a HetNet scenario is to partition the available spectrum into two separate component carriers and assign the primary component carriers (PCC) to different network layers as shown in figure 3.4 below.
control
data f1
f2
f1 f2 Macro
Femto
Figure 3.4: Illustration of Carrier aggregation in LTE
Primary component carriers is the cell that provides the control information to the UEs, by assigning this different frequencies interference on control channels between network layer can be avoided. By means of so called cross carrier scheduling each network layer can still schedule UEs on the other CCs called secondary component carriers (SCC) [24]
In the figure 3.4, f1 and f2 are PCC and SCC respectively where 5 sub-frames are shown in each carrier. These carrier’s components are used by both Macro layer and Femto layer. The sub-frames contain PDCCH, PCFICH and PHICH in the beginning the blue part which is control signal and green part is data signal.
As shown in figure 3.4 Macro layer can schedule its control signal on f1 but can still schedule its users on both f1 and f2 so by scheduling control and data information for both Macro and Femto layers on different component carriers, interference on control and data can be avoided.
As can be seen in the sub-frame 3 of figure 3.4, Macro layer has scheduled the user in the same frequency as Femto layer scheduling its user, this can be done when Femto user is center Femto user as the interference from Macro layer on center Femto user can be tolerated. If Femto user is in range extension region, Femto users are scheduled in different carrier then in Macro users. The disadvantage of carrier aggregation with cross carrier scheduling is that it is not backward compatible to release 8 and 9 terminals. It supports by release 10 terminals and onwards [14].
3.4.3 Almost Blank Sub-frame
The almost blank sub-frame is a sub-frame with reduced downlink transmission power and activity. In this, transmission from Macro eNB causing high interference onto Femto eNB users is periodically muted during entire sub-frames. Thus muted sub-frames are referred to as almost blank sub-frame. Using this approach, Femto cell users that are suffering from a high level of interference from the aggressor Macro eNB have a chance to be served. However, for the backward compatibility or to avoid radio link failure reasons certain signals needs to be transmitted in ABSs.
20 Such signals are:
Cell specific reference signals (CRS) which are also called pilots
SIB-1 and paging with their associated PDCCH
Physical broadcast channel (PBCH)
Primary and secondary synchronization channels (PSS/SSS)
Collisions of sub-frame muting with PSS, SSS, SIB-1 and paging should be minimized. PSS, SSS, SIB-1 and paging are transmitted in sub-frames #0, #1, #5 and #9.Without carefully handling of these channels in the ABSs significant performance degradation can occur in some situations.
ABSs muting patterns are configured semi-statically and signaled between eNBs over the X2 interface. These patterns are signaled in the form of bitmaps of length 40, i.e. spanning over 4 frames for FDD and 2 to 7 for TDD. Hence they can be configured dynamically by the network.
control data Macro
Femto
ABS ABS
Received SINR femto user
Time
Figure 3.5: Illustration of ABS in LTE
As shown in figure 3.5, Usage of ABS causes different level of interference between the sub- frames of Macro and Femto. If Femto user is in range extension region and it is interfered by strong Macro then Femto user can be served during the time when Macro transmit ABS else Femto user can be served during ABS or non-ABS.
Colliding and Non colliding CRS 3.5
As it is explained earlier for the sake of backward compatibility and to avoid radio link failure signals like CRSs, PCH, PBCH and PSS/SSS are still needs to be transmitted in the ABSs.
For simple study and investigation on the interference issue, we have considered a Macro cell transmitting ABS with only CRS and a Femto is transmitting a sub-frame with PDSCH channel and CRS. Since, the CRSs are spread all over the sub-frame in both in time and frequency domain there is a chances that CRS of Femto sub-frame and CRS of ABS collide, which is one of the worst scenarios encountered in the heterogeneous environment.
21
This situation is called colliding CRS case as shown in figure 3.6(B). As CRSs are primarily used for channel estimation, if they collide, Channel estimation quality won’t be good and system performance will degrade. The effect of interference from PCH, PBCH and PSS/SSS in ABS can still be controlled by not assigning the sub-frames where those channel appears as ABS, and also by using sub-frame shift between the Macro and Femto [1], which is sometime referred as non-colliding case shown in figure 3.6(A).
(A) Non Colliding Case (B) Colliding Case
Figure 3.6: Illustration of Colliding and Non Colliding CRS
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Chapter 4
Channel Model
Introduction to Air Interface 4.1
It is very important to have a good channel model for simulations as close to the reality as possible. Correct channel models are also essential for testing, parameter optimization, performance evaluation of communication systems and deployment of communication system for reliable transfer of information between transmitter and receiver. On the basis of time, channel may be time-invariant i.e. channel is constant over time or time-variant channel i.e.
channel varies over time. The time-invariant channels are relatively easy to estimate since it only requires one estimate at the beginning of the reception of the signal whereas time-variant channel is much harder to estimate since it requires continuous estimation.
The major complication in modeling of a wireless channel is due to complex propagation processes. Multiple copies of transmitted signal received from transmitter through different propagation mechanisms shown in figure 4.1.
Figure 4.1: Multipath effect [25]
The multipath propagation occurs due to the following reasons [24], [26].
Reflection, when electromagnetic waves get contact with flat and even surfaces then they are reflected
Diffraction, caused by bending of electromagnetic waves around corners of buildings and other objects.
Scattering, when electromagnetic waves get contact with the objects having uneven or irregular surface then they are get scattered.
Energy of the wave is partially absorbed when it passes through some special objects.
If channel is varying due to relative motion of large objects to that of radio device causes often slow fading, on the other hand multipath components of the signal that mix up either
23
constructively or destructively causes fast fading. As a result of the terminal movements on the order of half a wavelength a constructive superposition becomes destructive and vice versa.
As shown from Figure 4.1 a receiver, receives multiple copies via different paths from transmitter i.e. a multipath channel and is called as frequency selective channel hence, the channel varies in frequency. On the other hand, when receiver receives only one copy of the transmitted signal which results in a flat frequency fading, which in practice not the case often. Instead the radio signal is reflected by objects before it reaches the receiver [8] and can be modeled in time domain as
(4.1)
Where, and are stationary, statistically independent and real-valued Gaussian processes and are typical not white processes, but instead colored Gaussian.
4.1.1 Rayleigh distributed channel
The central limits theorem will model the channel as a Rayleigh distributed channel, when there are enough different paths, hence, and are Gaussian with zero-mean in equation (4.1). The Equation (4.1) in polar form is given below
(4.2)
Where
√ (4.3)
And
(4.4)
This distribution represents the sum of amplitudes of the same order of a large number of uncorrelated components with phases uniformly distributed in the interval (0, 2π). The power delay profile (PDF) of Rayleigh distributed channel is described by equation below
(4.5)
24
Where, [ ] [ ] and for < 0. The parameter is the root mean square value of the received signal.
4.1.2 Rician distributed channel
In the presence of line of sight (LOS) component, the channel can be modeled as a Rician distribution and is given by equation (4.6) as in [27].
( ) (4.6)
Where modified Bassel function of first kind and Zero order and is the amplitude of the line of sight component (LOS). In the absence of LOS component, Rician distribution can be simply expressed as Rayleigh distribution. Since in this thesis MIMO is implemented and it is assumed that there is no line of sight between the transmitter antennas and the receiver antenna, so Rician distributed channel model is not our interest here.
4.1.3 Maximum Doppler shift
In statistical analysis of an extended Clarke’s model, the time varying nature of the wireless channel is described by the wiener processes and where fixed Doppler shift through the multi- path signal is introduced to describe the situation of a moving receiver and transmitter. A closed-form expression for the autocorrelation function of the fading channel is derived which contain a zero order Bessel function and decaying factor. The Bessel is the result of the Doppler shift introduced due to the moving receiver and transmitter [28]. The maximum Doppler shift is given by
(4.7)
Where is the Doppler shift, is the carrier frequency, is the relative velocity and is the propagation speed of signal (speed of light) The above equation describes fast and slow fading in batter way, since the receiver and transmitter moving relative to each other is the main cause of fading.
Jake’s Channel model 4.2
In typical radio mobile communication scenario the transmitter is generally fixed whereas the receiver is moving and reflection through objects produces a multi-path signal effect. The direct LOS signal is not available in many cases and hence only signal reflected through objects reach the moving receiver. The total received signal at moving receiver is well defined by Jake’s model [29]. Equation (4.8) shows the impulse response of the channel;
∑ (4.8)
