Downlink Joint Resource Allocation with Adaptive Modulation and Coding

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Long Term Evolution (LTE) System

LTE Design Goals

The LTE system is designed by the way to offer enhancements obviously observed in the application layer building on the physical layer specifications. The high throughput is the major benefit ahead; in addition the spectrum flexibility, coverage improvement, very low latency, increased spectral-and energy-efficiency and other benefits are also provided. We describe some of that in what follows.

Long Term Evolution (LTE) System

High data rate
LTE system provides high data rate services due to the large bandwidth occupation, high order digital modulation utilization, up to 6 bits/symbol for the 64-QAM, link adaptation based on the channel status and adopting of the Multiple Input Multiple Output (MIMO) transmission scheme. These techniques and others are introduced offering a remarkable increasing of the data rate. Indeed, the peak data rate offered by the LTE in 20 MHz, reaches 100 Mbps (Mega bits per second) in Downlink transmission while the Uplink offers 50 Mbps.
Spectrum flexibility
The LTE system supports scalable bandwidths, 1.4 to 20 MHz as shown in Table 2.2. Moreover, the Orthoghonal Frequency Division Multiplexing (OFDMA) and the (Single Carrier-Frequency Division Multiple Access (SC-FDMA) techniques are the basis of the LTE system transmission (will be discussed later). These techniques are characterized by the high bandwidth flexibility and dynamically assignment to users; thus, they are adapted to the nomadic and mobile applications. In addition, LTE system supports both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD) to alternate between downlink and uplink traffics. This duality also offers spectrum flexibility.
Low latency
Latency is an important communication factor reflecting the QoS. Users’ consideration latency is defined as the time taken by the data packet to be transmitted from the UE’s data buffer and the serving gateway of the core network and vice versa. Comparing to the older telecommunication system (e.g. GPRS or EDGE) where the round-trip latencies of the data networks are ranged in the 600 − 700 ms, the LTE round-trip latencies are in the 50 ms range.
Improved coverage
Full performance services still provided for up to 5 km, and services are maintained with slight degradation for distance between 5 Km and 30 Km. The coverage improvement in addition to high performance transmission enhances the overall system capacity.

LTE Physical Layer Specifications

The LTE system requirements differ between the downlink (forward) and uplink (reversed) transmissions since the front-ends of the link: base station and UE terminal differ in sev-eral ways (e.g. power consumption, equipment deployment and sizes, BS sophisticated infrastructure)-Figure 2.1. We present in what follows the LTE specifications of both di-rections.

Downlink Direction Characteristics

The basic of the transmission in downlink is the Orthogonal Frequency Division Multi-plexing (OFDM) technique. A large number of orthogonal, overlapping, narrowband sub-channels (sub-carriers) are transmitted in parallel and divide the available transmission bandwidth. Doing that, the multipath fading effect is combated and so, the Inter-Symbol Interference (ISI) problem is mitigated. Indeed, for each sub-channel the channel status is considered as flat. This technique is based on the Fast Fourier Transform (FFT) of the information data. In order to mitigate the residue interference and the Inter-Carrier Inter-ference (ICI), a Cyclic Prefix (CP) is added to the head of modulated stream. The CP is a number of symbols copied from the tail, so we obtain the OFDM symbol. Consequently, the fading effect of the channel is overcoming without need to complex equalizers and ex-pansive components at the receiver front-end. In downlink, where the receiver is the User Equipment (UE), this fact is very suitable since the power consumption and the terminal cost are notably reduced.
For multi-user access, the Orthogonal Frequency Division Multiple Access (OFDMA) is adopted as a basis of the LTE multi-access Downlink transmission. The media access is the time-frequency domain as represented in Figure 2.2. The smallest unit to allocate is called a Resource Element (RE) represented by one subcarrier during one OFDM period. Each user is allocated a determined number of Resource Blocks (RBs). In LTE system, each RB is composed from 6 or 7 OFDM symbols forming one slot and carried upon “12” subcarriers spaced by “15KHz”. The number of OFDM symbols in each slot depends on the CP length. Thus, the allocation granularity is “180 KHz” in frequency domain and a one slot of “0.5 msec” in time domain. Two CP types are considered: normal and extended, depending on the channel delay spread. For highly spreading environments, the extended CP is used and one slot corresponds to “6” OFDM symbols instead of “7” OFDM symbols for normal CP.
The OFDMA is a multi-carrier transmission scheme that enhances the spectral efficiency. However, as will be described later, this scheme presents some limitations. Especially, the OFDM technique suffers from high level Peak-to-Average Power Ratio (PAPR). This is due to the fact that each OFDM sub-carrier is independently modulated and as known each modulation scheme presents a different symbol power. Indeed, a high PAPR im-poses high power consumption at the transmitter front-end and sophisticated non-linear amplifiers. This fact does not meet with the small size constraint of the UE in the Uplink transmission. Therefore, for the uplink, an alternative transmission technique is adopted by the LTE standard in order to overcome the high PAPR effect, however presenting other limitations.

Uplink Direction Characteristics

In the uplink, the User Equipment (UE) transmits data by using a pre-determined number of RBs in the same manner described for the downlink transmission in Figure 2.2. The difference is by the multi-access modulation scheme. The Single Carrier-Frequency Division Multiple Access (SC-FDMA) is the multiple access scheme currently adopted for the uplink in the 3GPP LTE system. The reason behind is the need to the highly ’power-efficient’ transmission for the UE, enabling improved coverage and reduced equipment complexity. In the LTE standard, the SC-FDMA is based on the Discrete Fourier Transform (DFT)-precoded OFDM that presents smaller PAPR than conventional OFDM. Thus, during each Resource Element (RE) a DFT-precoded OFDM symbol is transmitted instead of the OFDM symbol. The SC-FDMA has a similar performance as conventional OFDMA and it offers the same degree of multipath mitigation. This scheme is actually a hybrid format that combines the low peak to average ratio provided by single-carrier systems with the multi-path interference resilience and flexible sub-carrier frequency allocation that OFDM provides.

Duplex schemes and framing

The duplexing is the mode adopted to differentiate between the downlink and uplink traffics. The scheduler at the base station controls both transmissions. The FDD is operated on a paired spectrum and the TDD on an unpaired spectrum. In addition, FDD deployment prepares the way for the 3G services while the TDD is matched for the evolution to the Time Division-Synchronous Code Division Multiple Access (TD-SCDMA).
The 3GPP LTE standard is designed in manner to support both duplex schemes: the Frequency Division Duplex (FDD) and the Time Division Duplex (TDD) also called TD-LTE [9]. The duplex duality offers spectrum flexibility according to the spectrum allocation and it simplifies the implementation of different standards with different duplex modes [10]. Although the physical layer processing is closely similar for FDD and TDD, these two schemes mainly differ by the transmitted frame structure. Thus, we consider two frame types: “type 1” for FDD duplex mode and “type 2” for TDD duplex mode as displays in Figure 2.3.
• FDD frame structure: One radio FDD frame is transmitted during 10 ms and it is composed to 10 subframes of 1 ms duration each one. Each subframe is divided to two slots of 0.5 ms. The uplink and the downlink are transmitted simultaneously but each transmission over a specified frequency band with respect to a set of FDD scheme configurations. This duplex scheme is complex due to the synchronization requirements.
• TDD frame structure: As in the FDD case, there is 10 subframes or 20 slots that constitutes the 10 ms total frame. For the TDD scheme, the guard period is widely necessary in order to avoid overlapping between the Downlink and Uplink transmis-sions. Thus, a ’special subframe’ represents this guard period. This subframe is divided to three parts: a Downlink part (DwPTS), a guard part (GP) and an Uplink part (UpPTS), as mentioned in Figure 2.3. Since the Downlink and Uplink share the same frame, the two transmissions occur simultaneously within the active cell. Generally the uplink and downlink traffics are not symmetrical depending on the data services and demands.

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Femto-Small Cells Networks

Alternatively to be served by the central far-away macrocell base station, the subscriber can be easily connected to a small femtoCell base station also referred to the eNodeB that it independently deployed whenever and wherever he desired. The eNodeB also known as femtocell access point (FAP) is a low-power access point, based on mobile technology, providing wireless connections to customers suffering from coverage issues in confined en-vironments or spectrum capacity problems in high density urban areas. Thus, the FAP acts as a cellular base station designed for use in indoor environments [11]. Nowadays, the major constructors develop modules that can be directly plugged onto existing Wi-Fi hotspots and act as a standard LTE base station in an indoor location. All FAPs are connected to a femtocell gateway where the traffic is transmitted through the operator’s core network [12]. The limited coverage area of a FAP allows a smaller number of users to take full advantage of the available spectrum.
Femtocells can serve simultaneously up to 10 or 20 users and deliver higher data rate connections depending on the transmission technology.

Femtocells’ benefits

We present here some of the major benefits of femtocells and their impact on the network.
Transparent cellular standards support – Femtocells are detected by the end de-vices as standard cellular base stations and user equipments can be connected seamlessly independently of the technology: LTE, WiMAX, GSM, UMTS, CDMA, WCDMA and many other existing and future standardized protocols.
Low RF power and energy efficiency – Unlike Wi-Fi access points, the transmit power of femtocells is between 10 and 100 milli-watts and is automatically adjusted over each resource block. Additionally, due to the short range, the mobile device can transmit at lower power than needed to reach the macrocell base station. Thus, the energy efficiency is improved with a better battery consumption.
Capacity – The low-power connection and the short range transmission limit the femto-cell coverage area. This allows a higher spectrum reuse and increases the network capacity. Moreover, users that cannot connect to the macrocell can be offloaded and served by the femtocell; This provides better experience for the mobile users and enhances the overall system capacity.
Auto-configuration and Cost benefits – The most important aspect of the femto-cell is operation cost reduction. From the operator’s perspective, FAP should to be simple to install and connect [13]. The zero configuration aspect of femtocells make it very con-venient and cost effective for a massive deployment especially in dense areas. In fact, no network planning or special configuration is required as in the macrocell base stations.

Femtocells’ Challenges

Many challenges arises from the femtocells deployment. Some of them are particular to small cells and some are a depending on the cellular technology. In our case, considering the LTE scheme, we differentiate several challenges [14].
Access method and handoff- Three access modes are configured by the femtocells:
open access, closed access and hybrid access [15].
For open access, all users can be connected to the femtocell, fact that increases the overall network capacity when the macrocell coverage is deficient. However, this method increases the number of signaling and applies a security issues.
The closed access method allows service for only home (indoor) environment users [16]. As a drawback, this method increases the interference to the macrocell since the femto-user radiation leaks from windows and/or doors.
In the hybrid access scenario, femtocell allows nonsubscribers to be connected, but with a limited amount of shared subchannels and with less priorities.
The choice to adopt depends on the transmission scheme to be used and the particularity and density of the deployment area.
Timing and synchronization- Perfect time synchronization is essential to avoid multi-access interference: cross-tier between macrocells and femtocells and co-tier between femtocells when uplink period of some cells overlaps with the downlink of others in TDD scheme. Since the centralization management is absent regarding the self-deployment of the ad-hoc location femtocells, the transmission synchronization becomes difficult. In ad-dition, high-precision oscillator’s equipments are so complex to be integrated in low-cost femtocells. Thus, the need to efficient approaches to achieve synchronization emerges.
Mobility management- When mobile user equipment enters in the coverage area of a new cell, it automatically triggers its connection to that cell.
For the femtocell, this handoff mechanism occurs frequently and more than in the macrocell case due to the small area coverage of the femtocell. This fact significantly increases the network signaling especially for open access mode. Consequently, reliable handoff manage-ment methods are needed to deal with these issues.
Interference management- The deployment of the femtocell by the end-user in-dependently from the centralize macrocell operator, introduces interference between the macrocell and the femtocells (cross-tier interference). Moreover, the installation the fem-tocell at the vicinity of another one leads into co-tier interference.
This issue is the major problem facing constructors for an effective deployment of this tech-nology. Thus, interference avoidance or cancellation techniques must be applied in order to manage allocation of resources and successfully exploit the femtocell network. This chal-lenge is firstly treated in this thesis where an interference mitigation approach is proposed for the forward (downlink) and reversed (uplink) links under the channel conditions and user requirements constraints.

Table of contents :

1 Introduction 
1.1 Problem Statement
1.2 Thesis Contributions
1.2.1 Adaptive Modulation and Coding for QoS-based Femtocell Resource Allocation with Power Control: AMC-QRAP Approach OFDMA-based Downlink AMC-QRAP SC-FDMA-based Uplink AMC-QRAP
1.2.2 Physical Layer LTE Enhancements Wavelet-based OFDM Multicarrier Transmission Approach Wavelet-based Edge Detection for Spectrum Sensing
1.3 Thesis Outline
2 State of The Art 
2.1 Introduction
2.2 Long Term Evolution (LTE) System
2.2.1 LTE Design Goals
2.2.2 LTE Physical Layer Specifications Downlink Direction Characteristics Uplink Direction Characteristics Duplex schemes and framing
2.3 Femto-Small Cells Networks
2.3.1 Femtocells’ benefits
2.3.2 Femtocells’ Challenges
2.4 Fundamental Wireless Communication Notions
2.5 Link Adaptation Issue
2.6 Literature Review
2.6.1 Downlink resource allocation approaches
2.6.2 Uplink resource allocation approaches
2.6.3 Wavelet-based signal processing enhancements Alternative wavelet-based OFDM approaches Spectrum Sensing techniques
2.7 Conclusion
3 Downlink Joint Resource Allocation with Adaptive Modulation and Coding
3.1 Introduction
3.2 System Description and Notations
3.2.1 Network Model
3.2.2 Propagation Model
3.2.3 Notations
3.3 Adaptive Modulation and Coding Concept
3.3.1 Definition
3.3.2 Modulation & Coding Scheme and Link Quality
3.3.3 Fixed Modulation and Coding (FMC) vs. Adaptive Modulation and Coding (AMC)
3.4 Downlink OFDMA AMC-based Joint Resource Allocation Proposal
3.4.1 Problem Formulation
3.4.2 Problem Resolution
3.5 Performance Metrics
3.5.1 Throughput Satisfaction Rate (TSR)
3.5.2 Spectrum Spatial Reuse (SSR)
3.5.3 Rate of rejected users
3.5.4 Average channel efficiency
3.5.5 Transmission power
3.6 Performance Evaluation
3.7 Conclusion
4 Uplink Joint Resource Allocation with Adaptive Modulation and Coding 
4.1 Introduction
4.2 SC-FDMA Transmission Mode
4.2.1 What is SC-FDMA and Why using it?
4.2.2 SC-FDMA v.s. OFDMA Block Diagram and Symbol Transmission SC-FDMA and OFDMA PAPR Comparison
4.3 System Description
4.3.1 System and Transmission Model
4.3.2 Notations
4.4 Uplink AMC-based Joint Resource Allocation Proposal
4.4.1 Problem Formulation Uplink Interference Scenarios Spectrum Sensing Phase Resource Allocation Phase
4.4.2 Problem Resolution
4.5 Performance Metrics
4.5.1 Throughput Satisfaction Rate (TSR)
4.5.2 Rate of rejected users
4.5.3 Spectrum Spatial Reuse (SSR)
4.5.4 Transmission power
4.5.5 Fairness
4.6 Performance Evaluation
4.7 Conclusion
5 Wavelet-based LTE Physical Layer Enhancements 
5.1 Introduction
5.2 Wavelet Signal Processing Tool
5.2.1 Fourier Transform: Analysis and Limitations
5.2.2 Short Term Fourier Transform- STFT
5.2.3 Wavelet Transform: Multi-Resolution Analysis Definition and Characteristics Types of the Wavelet Transform Wavelet in Communications and Application fields
5.3 Wavelet-based OFDM Multicarrier Approach
5.3.1 Fourier-based OFDM Limitations
5.3.2 Wavelet Orthogonal basis for Multicarrier Transmission
5.3.3 Wavelet-based OFDM Alternative System
5.3.4 How the wavelet alleviates the Fourier-based OFDM problems?
5.3.5 Simulation Results and Comparison
5.4 Automatic Wavelet-based Edge Detection for Spectrum Sensing
5.4.1 Edge Detection Wavelet Property
5.4.2 Wavelet-based Spectrum Sensing Approach System Model Approach Description Automatic Local Maxima Detection
5.5 Wavelet Applications for the LTE Mobile System
5.5.1 5th Generation Roadmap
5.5.2 Wavelet-based Downlink Enhancement
5.5.3 Wavelet-based Uplink Enhancement
5.6 Conclusion
6 Conclusion and Future Works 
6.1 Conclusion
6.2 Future works and perspectives
7 List of Publications 


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