1 Introduction

The convenience and popularity of wireless technology has now extended into multimedia communications, where it poses a unique challenge for transmitting high rate voice, image, and data signals simultaneously, synchronously, and virtually error-free . That challenge is currently being met through Orthogonal Frequency Division Multiplexing (OFDM), an interface protocol that divides incoming data streams into sub-streams with overlapping frequencies that can then be transmitted in parallel over orthogonal subcarriers [2,3]. To allow multiple accesses in OFDM , Orthogonal Frequency Division Multiple Access (OFDMA) was introduced. Relaying techniques, along with OFDMA, are used to achieve high data rate and high spectral efficiency.

1.1 Orthogonal Frequency Division Multiple Access

OFDMA, an interface protocol combining features of OFDM and frequency division multiple access (FDMA)., was developed to move OFDM technology from a fixed-access wireless system to a true cellular system with mobility with same underlying technology, but more flexibility was defined in the operation of the system [1,8]. In OFDMA, subcarriers are grouped into larger units, referred to as sub-channels, and these sub-channels are further grouped into bursts which can be allocated to wireless users [4].

1.2 Relay-Enhanced Networks

In cellular systems, a way to achieve remarkable increase in data rate, but without claiming for more bandwidth, is to shrink cell sizes, however, with smaller cells more base stations (BS's) are needed to cover a same area due to which deployment and networking of new BS's acquire significant costs [5]. An alternative solution to this problem is to deploy smart relay stations (RS's), which can communication with each other and with BS's through wireless connections reducing system's cost. A relay station (RS), also called repeater or multi-hop station, is a radio system that helps to improve coverage and capacity of a base station (BS) and the resulting networks employing relay stations are sometimes called cooperative networks [6].

1.3 Technological Requirement

The continuously evolving wireless multimedia services push the telecommunication industries to set a very high data rate requirement for next generation mobile communication systems. As spectrum resource becomes very scarce and expensive, how to utilize this resource wisely to fulfil high quality user experiences is a very challenging research topic. Orthogonal frequency-division multiple access (OFDMA)-based RRM schemes together with relaying techniques allocate different portions of radio resources to different users in both the frequency and time domains and offers a promising technology for providing ubiquitous high-data-rate coverage with comparatively low cost than deploying multiple base stations [5].

Although wireless services are the demand of future due to their mobility and low cost infrastructure but along with this they suffer serious channel impairments. In particular, the channel suffers from frequency selective fading and distance dependent fading (i.e., large-scale fading) [1, 8]. While frequency selective fading results in inter-symbol-interference (ISI), large-scale fading attenuates the transmitted signal below a level at which it can be correctly decoded. Orthogonal Frequency-Division Multiple Access (OFDMA) relay-enhanced cellular network, the integration of multi-hop relaying with OFDMA infrastructure, has become one of the most promising solutions for next-generation wireless communications.

1.3.1 Frequency Selective Fading

In wireless communications, the transmitted signal is typically reaching the receiver through multiple propagation paths (reflections from buildings, etc.), each having a different relative delay and amplitude. This is called multipath propagation and causes different parts of the transmitted signal spectrum to be attenuated differently, which is known as frequency-selective fading. In addition to this, due to the mobility of transmitter and/or receiver or some other time-varying characteristics of the transmission environment, the principal characteristics of the wireless channel change in time which results in time-varying fading of the received signal [9].

1.3.2 Large Scale Fading

Large scale fading is explained by the gradual loss of received signal power (since it propagates in all directions) with transmitter-receiver (T-R) separation distance.

These phenomenons's cause attenuation in the signal and decrease in its power. To overcome this we use diversity and multi-hop relaying.

1.3.3 Diversity

Diversity refers to a method for improving the reliability of a message signal by using two or morecommunication channelswith different characteristics. Diversity plays an important role in combatingfadingandco-channel interferenceand avoidingerror bursts. It is based on the fact that individual channels experience different levels of fading and interference. Multiple versions of the same signal may be transmitted and/or received and combined in the receiver [10].

1.4 Proposed Simulation Model

We developed a simulation model in which each user-pair is allocated dynamically a pair of relay and subcarrier in order to maximize its achievable sum-rate while satisfying the minimum rate requirement. The algorithm and the results of the simulation model are given in chapter 4.

1.5 Objectives

The objective of our project is to have a detail overview of the literature regarding Orthogonal Frequency Division Multiple Access (OFDMA), Radio Resource Management (RRM) and Relaying techniques. After literature review we developed a simulation framework in which we will try to use minimum resources to get maximum throughput by using dynamic resource allocation.

1.6 Tools

For the design and implementation of proposed Algorithm, we have used the following tools

  • MATLAB
  • Smart Draw
  • Corel Draw

1.7 Overview

Chapter 2 contains the literature review. It explains the basic principles of OFDMA, Radio Resource Management (RRM) and the relaying techniques.

Chapter 3 explains the implementation of OFDM generation and reception that how an OFDM signal is generated and transmitted through the channel and how it is recovered at the receiver.

Chapter 4 could be considered as the main part of thesis. It focuses on the simulation framework and the code. We have followed the paper “Subcarrier Allocation for multiuser two-way OFDMA Relay networks with Fairness Constraints”. In this section we have tried to implement the Dynamic Resource Allocation algorithm in order to achieve the maximum sum rate. Results are also discussed at the end of the end of the chapter.

2 Literature Review

Introduction:

First section of this Chapter gives a brief overview about OFDMA.OFDMA basically is the combination of Orthogonal Frequency Division Multiplexing (OFDM) and Frequency Division Multiplexing Access (FDMA).OFDMA provides high data rates even through multipath fading channels. In order to understand OFDMA, we must have brief introduction to Modulation, Multiple Access, Propagation mechanisms, its effects and its impairments while using OFDMA.

2.1 Modulation

Modulation is the method of mapping data with change in carrier phase, amplitude, frequency or the combination [11]. There are two types of modulation techniques named as Single Carrier Modulation (SCM) Transmission Technique or Multicarrier Modulation (MCM) Transmission Technique. [12]

Single Carrier Modulation (SCM)

In single carrier transmission modulation (SCM) transmission, information is modulated using adjustment of frequency, phase and amplitude of a single carrier [12].

Multi Carrier Modulation (MCM)

In multicarrier modulation transmission, input bit stream is split into several parallel bit streams then each bit stream simultaneously modulates with several sub-carriers (SCs) [12].

2.2 Multiplexing

Multiplexing is the method of sharing bandwidth and resources with other data channels. Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end [13].

2.2.1 Analog Transmission

In analog transmission, signals are multiplexed using frequency division multiplexing (FDM), in which the carrier bandwidth is divided into sub channels of different frequency widths,and each signal is carried at the same time in parallel.

2.2.2 Digital Transmission

In digital transmission, signals are commonly multiplexed using time-division multiplexing (TDM), in which the multiple signals are carried over the same channel in alternating time slots.

2.2.3 Need for OFDMA

General wireless cellular systems are multi-users systems. We have limited radio resources as limited bandwidth and limited number of channels. The radio resources must be shared among multiple users. So OFDM is a better choice in this case. OFDM is the combination of modulation and multiplexing. It may be a modulation technique if we analyze the relation between input and output signals. It may be a multiplexing technique if we analyze the output signal which is the linear sum of modulated signal. In OFDM the signal is firstly split into sub channels, modulated and then re-multiplexed to create OFDM carrier. The spacing between carriers is such that they are orthogonal to one another. Therefore there is no need of guard band between carriers. In this way we are saving the bandwidth and utilizing our resources efficiently.

2.3 Radio Propagation Mechanisms

There are 3 propagation mechanisms: Reflection, Diffraction and Scattering. These 3 phenomenon cause distortion in radio signal which give rise to propagation losses and fading in signals [14].

2.3.1 Reflection

Reflection occurs when a propagating Electro-Magnetic (EM) wave impinges upon an object which has very large dimensions as compared to the wavelength of the propagating wave. Reflections occur from the surface of the earth and from buildings and walls.

2.3.2 Diffraction

When the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities (edges), diffraction occurs. The secondary waves resulting from the obstructing surface are present throughout the space and even behind the obstacle, giving rise to a bending of waves around the obstacle, even when a line-of-sight path does not exist between transmitter and receiver. At high frequencies, diffraction, like reflection, depends on the geometry of the object, as well as the amplitude, phase and polarization of the incident wave at the point of diffraction.

2.3.3 Scattering

When the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength, and where the number of obstacles per unit volume is large. Scattered waves are produced by rough surfaces, small objects or by other irregularities in the channel. In practice, foliage, street signs and lamp posts produce scattering in a mobile radio communications system.

2.4 Effects of Radio Propagation Mechanisms

The three basic propagation mechanisms namely reflection, diffraction and scattering as we have explained above affect on the signal as it passes through the channel. These three radio propagation phenomena can usually be distinguished as large-scale path loss, shadowing and multipath fading [14][15].

2.4.1 Path Loss

Path Lossis the attenuation occurring by an electromagnetic wave in transit from a transmitter to a receiver in a telecommunication system. In simple words, it governs the deterministic attenuation power depending only upon the distance between two communicating entities. It is considered as large scale fading because it does not change rapidly.

2.4.2 Shadowing

Shadowingis the result of movement of transmitter, receiver or any channel component referred to as (obstacles). Shadowing is a statistical parameter. Shadowing follows a log-normal distribution about the values governed by path loss. Although shadowing depends heavily upon the channel conditions and density of obstacles in the channel, it is also normally considered a large scale fading component alongside path loss.

2.4.3 Multipath Fading

Multipath Fadingis the result of multiple propagation paths which are created by reflection, diffraction and scattering. When channel has multiple paths. Each of the paths created due to these mechanisms may have its characteristic power, delay and phase. So receiver will be receiving a large number of replicas of initially transmitted signal at each instant of time. The summation of these signals at receiver may cause constructive or destructive interferences depending upon the delays and phases of multiple signals. Due to its fast characteristic nature, multipath fading is called small scale fading.

2.5 Orthogonal Frequency Division Multiplexing (OFDM)

Orthogonal Frequency Division Multiplexing (OFDM) is an efficient multicarrier modulation that is robust to multi-path radio channel impairments [15]. Now-a-days it is widely accepted that OFDM is the most promising scheme in future high data-rate broadband wireless communication systems.

OFDM is a special case of MCM transmission. In OFDM, high data rate input bit stream or data is first converted into several parallel bit stream, than each low rate bit stream is modulated with subcarrier. The several subcarriers are closely spaced. However being orthogonal they do not interfere with each other.

2.5.1 Orthognality

Signals are orthogonal if they are mutually independent of each other. Orthogonality is a property that allows multiple information signals to be transmitted perfectly over a common channel and detected, without interference. Loss of orthogonality results in blurring between these information signals and degradation in communications. Many common multiplexing schemes are inherently orthogonal.

The term OFDM has been reserved for a special form of FDM. The subcarriers in an OFDM signal are spaced as close as is theoretically possible while maintain orthogonality between them.In FDM there needs a guard band between channels to avoid interference between channels. The addition of guard band between channels greatly reduces the spectral efficiency. In OFDM, it was required to arrange sub carriers in such a way that the side band of each sub carrier overlap and signal is received without interference. The sub-carriers (SCs) must be orthogonal to each other, which eliminates the guard band and improves the spectral efficiency .

2.5.2 Conditions of orthogonality

2.5.2.1 Orthogonal Vectors

Vectors A and B are two different vectors, they are said to be orthogonal if their dot product is zero

2.6 OFDM GENERATION AND RECEPTION

OFDM signals are typically generated digitally due to the complexity of implementation in the analog domain. The transmission side is used to transmit digital data by mapping the subcarrier amplitude and phase. It then transforms this spectral representation of the data into the time domain using an Inverse Discrete Fourier Transform (IDFT) but due to much more computational efficiency in Inverse Fast Fourier Transform (IFFT), IFFT is used in all practical systems.

The receiver side performs the reverse operations of the transmission side, mixing the RF signal to base band for processing, and then a Fast Fourier Transform (FFT) is employed to analyze the signal in the frequency domain. The demodulation of the frequency domain signal is then performed in order to obtain the transmitted digital data.

The IFFT and the FFT are complementary function and the most suitable term depends on whether the signal is being recovered or transmitted but the cases where the signal is independent of this distinction then these terms can be used interchangeably [15].

2.6.1 OFDM Block Diagram

2.6.2 Implementation of OFDM Block Diagram

2.6.2.1 Serial to Parallel Conversion:

In an OFDM system, each channel can be broken down into number of sub-carriers. The use of sub-carriers can help to increase the spectral efficiency but requires additional processing by the transmitter and receiver which is necessary to convert a serial bit stream into several parallel bit streams to be divided among the individual carriers. This makes the processing faster as well as is used for mapping symbols on sub-carriers.

2.6.2.2 Modulation of Data:

Once the bit stream has been divided among the individual sub-carriers by the use of serial to parallel converter, each sub-carrier is modulated using 16 QAM scheme as if it was an individual channel before all channels are combined back together and transmitted as a whole.

2.6.2.3 Inverse Fourier Transform:

The role of the IFFT is to modulate each sub-channel onto the appropriate carrier thus after the required spectrum is worked out, an inverse Fourier transform is used to find the corresponding time domain waveform.

2.6.2.4 Parallel to Serial Conversion:

Once the inverse Fourier transform has been done each symbol must be combined together and then transmitted as one signal. Thus, the parallel to serial conversion stage is the process of summing all sub-carriers and combining them into one signal

2.6.2.5 Channel:

The OFDM signal is then transmitted over a channel with AWGN having SNR of 10 dB.

2.6.2.6 Receiver:

The receiver basically does the reverse operations to the transmitter. The FFT of each symbol is taken to find the original transmitted spectrum. The phase angle of each transmission carrier is then evaluated and converted back to the data word by demodulating the received phase. The data words are then combined back to the same word size as the original data.

2.7 OFDMA in a broader perspective

OFDM is a modulation scheme that allows digital data to be efficiently and reliably transmitted over a radio channel, even in multipath environments [17]. OFDM transmits data by using a large number of narrow bandwidth carriers. These carriers are regularly spaced in frequency, forming a block of spectrum. The frequency spacing and time synchronization of the carriers is chosen in such a way that the carriers are orthogonal, meaning that they do not interfere with each other. This is despite the carriers overlapping each other in the frequency domain [18]. The name ‘OFDM' is derived from the fact that the digital data is sent using many carriers, each of a different frequency (Frequency Division Multiplexing) and these carriers are orthogonal to each other [19].

2.7.1 History of OFDMA

The origins of OFDM development started in the late 1950's with the introduction of Frequency Division Multiplexing (FDM) for data communications. In 1966 Chang patented the structure of OFDM and published the concept of using orthogonal overlapping multi-tone signals for data communications. In 1971 Weinstein introduced the idea of using a Discrete Fourier Transform (DFT) for Implementation of the generation and reception of OFDM signals, eliminating the requirement for banks of analog subcarrier oscillators. This presented an opportunity for an easy implementation of OFDM, especially with the use of Fast Fourier Transforms (FFT), which are an efficient implementation of the DFT. This suggested that the easiest implementation of OFDM is with the use of Digital Signal Processing (DSP), which can implement FFT algorithms. It is only recently that the advances in integrated circuit technology have made the implementation of OFDM cost effective.

The reliance on DSP prevented the wide spread use of OFDM during the early development of OFDM. It wasn't until the late 1980's that work began on the development of OFDM for commercial use, with the introduction of the Digital Audio Broadcasting (DAB) system .

2.7.2 Advantages using OFDMA

There are some advantages using OFDMA.

Ø OFDM is a highly bandwidth efficient scheme because different sub-carriers are orthogonal but they are overlapping.

Ø Flexible and can be made adaptive; different modulation schemes for subcarriers, bit loading, adaptable bandwidth/data rates possible.

Ø Has excellent ICI performance because of addition of cyclic prefix.

Ø In OFDM equalization is performed in frequency domain which becomes very easy as compared to the time domain equalization.

Ø Very good at mitigating the effects of delay spread.

Ø Due to the use of many sub-carriers, the symbol duration on the sub-carriers is increased, relative to delay spread.

Ø ISI is avoided through the use of guard interval.

Ø Resistant to frequency selective fading as compared to single carrier system.

Ø Used for high data rate transmission.

Ø OFDMA provides flexibility of deployment across a variety of frequency bands with little need for modification is of paramount importance.

Ø A single frequency network can be used to provide excellent coverage and good frequency re-use.

Ø OFDMA offers frequency diversity by spreading the carriers all over the used spectrum.

2.7.3 Challenges using OFDMA

These are the difficulties we have to face while using OFDMA [20][21][22],

Ø The OFDM signal suffers from a very high peak to average power ratio (PAPR) therefore it requires transmitter RF power amplifiers to be sufficiently linear in the range of high input power.

Ø Sensitive to carrier frequency offset, needs frequency offset correction in the receiver.

Ø Sensitive to oscillator phase noise, clean and stable oscillator required.

Ø The use of guard interval to mitigate ISI affects the bandwidth efficiency.

Ø OFDM is sensitive to Doppler shift - frequency errors offset the receiver and if not corrected the orthogonality between the carriers is degraded.

Ø If only a few carriers are assigned to each user the resistance to selective fading will be degraded or lost.

Ø It has a relatively high sensitivity to frequency offsets as this degrades the orthogonality between the carriers.

Ø It is sensitive to phase noise on the oscillators as this degrades the orthogonaility between the carriers.

2.7.4 Comparison with CDMA in terms of benefits

2.7.4.2 CDMA Advantages:

CDMA has some advantages over OFDMA [22],

Ø Not as complicated to implement as OFDM based systems.

Ø As CDMA has a wide bandwidth, it is difficult to equalise the overall spectrum - significant levels of processing would be needed for this as it consists of a continuous signal and not discrete carriers.

Ø Not as easy to aggregate spectrum as for OFDM.

2.7.5 OFDMA in the Real World:

UMTS, the European standard for the 3G cellular mobile communications, and IEEE 802.16, a broadband wireless access standard for metropolitan area networks (MAN), are two live examples for industrial support of OFDMA. Table 1 shows the basic parameters of these two systems.

UMTS(Cellular )

IEEE IEEE 802.16 ( Wireless I IEEE 802.16 ( Wireless MAN )

System bandwidth

100kHz-1.6MHz (Flexible)

6Mhz

Number of subcarriers

240 / 100kHz

2048

Subcarrier spacing

4.16kHz

3.35kHz

n Subcarriers / Band-unit

24 Subcarrier/Bandslot

53 Subcarrier/Subchannel

Modulation time

240 µs

298 µs

Guard time

38 µs (pre-) and 8 µs (post-guard)

38 µs

Symbol time

288 µs

340 µs

Resource allocation unit

1 bandslot, 1 timeslot (1 symbol)

1 Subchannel, 1 timeslot

Modulation

QPSK , 8-PSK

QPSK, 16-QAM, 64-QAM

(differential and coherent)

Channel coding

Convolutional (1/3, 2/3)

Turbo (1/2)

Opt. Outer Reed-Solomon (4/5)

Frequency hopping

1 hop/burst, 876 hop/sec, 1.6MHz

NA????????

(Flexible)

Max. Data throughput

11

Table 1. OFDMA system parameters in the UMTS and IEEE 802.16 standards

2.8 Radio Resource Management

In second section of this chapter we will discuss radio resource management schemes, why we need them and how they improve the efficiency of the network. Radio resource management is the system level control of co-channel interference and other radio transmission characteristics in wireless communication systems. Radio resource management involves algorithms and strategies for controlling parameters such as

Ø Transmit power

Ø Sub carrier allocation

Ø Data rates

Ø Handover criteria

Ø Modulation scheme

Ø Error coding scheme, etc

2.8.1 Study of Radio Resource Management

End-to-end reconfigurability has a strong impact on all aspects of the system, ranging from the terminal, to the air interface, up to the network side. Future network architectures must be flexible enough to support scalability as well as reconfigurable network elements, in order to provide the best possible resource management solutions in hand with cost effective network deployment. The ultimate aim is to increase spectrum efficiency through the use of more flexible spectrum allocation and radio resource management schemes, although suitable load balancing mechanisms are also desirable to maximize system capacity, to optimize QoS provision, and to increase spectrum efficiency. Once in place, mobile users will benefit from this by being able to access required services when and where needed, at an affordable cost. From an engineering point of view, the best possible solution can only be achieved when elements of the radio network are properly configured and suitable radio resource management approaches/algorithms are applied. In other words, the efficient management of the whole reconfiguration decision process is necessary, in order to exploit the advantages provided by reconfigurability. For this purpose, future mobile radio networks must meet the challenge of providing higher quality of service through supporting increased mobility and throughput of multimedia services, even considering scarcity of spectrum resources. Although the size of frequency spectrum physically limits the capacity of radio networks, effective solutions to increase spectrum efficiency can optimize usage of available capacity.

Through inspecting the needs of relevant participants in a mobile communication system, i.e., the Terminal, User, Service and Network, effective solutions can be used to define the communication configuration between the Terminal and Network, dependent on the requirements of Services demanded by Users. In other words, it is necessary to identify proper communications mechanisms between communications apparatus, based on the characteristics of users and their services. This raises further questions about how to manage traffic in heterogeneous networks in an efficient way.

2.8.2 Methods of RRM

2.8.2.1 Network based functions

Ø Admission control (AC)

Ø Load control (LC)

Ø Packet scheduler (PS)

Ø Resource Manager (RM)

Admission control

  1. In the decision procedure AC will use threshold form network planning and from

Interference measurements.

  1. The new connection should not impact the planned coverage and quality of existing
  2. Connections. (During the whole connection time.)
  3. AC estimates the UL and DL load increase which new connection would produce.
  4. AC uses load information from LC and PC.
  5. Load change depends on attributes of RAB: traffic and quality parameters.
  6. If UL or DL limit threshold is exceeded the RAB is not admitted.
  7. AC derives the transmitted bit rate, processing gain, Radio link initial quality parameters, target BER, BLER, Eb/No, SIR target.
  8. AC manages the bearer mapping
  9. The L1 parameters to be used during the call.
  10. AC initiates the forced call release, forced inter-frequency or intersystem handover.

Load control

Reason of load control

Optimize the capacity of a cell and prevent overload

Ø The interference main resource criteria.

Ø LC measures continuously UL and DL interference.

Ø RRM acts based on the measurements and parameters from planning

Preventive load control

Ø In normal conditions LC takes care that the network is not overloaded and remains

Stable.

Overload condition .

Ø LC is responsible for reducing the load and bringing the network back into operating area.

Fast LC actions in BTS

Ø Lower SIR target for the uplink inner-loop PC.

Ø LC actions located in the RNC.

Ø Interact with PS and throttle back packet data traffic.

Ø Lower bit rates of RT users.(speech service or CS data).

Ø WCDMA interfrequency or GSM intersystem handover.

Ø Drop single calls in a controlled manner.

2.8.2.3 Connection based functions

Ø Handover Control (HC)

Ø Power Control (PC)

Power control

Ø Uplink open loop power control.

Ø Downlink open loop power control.

Ø Power in downlink common channels.

Ø Uplink inner (closed) loop power control.

Ø Downlink inner (closed) loop power control.

Ø Outer loop power control.

Ø Power control in compressed mode.

Handover

Ø Intersystem handover.

Ø Intrafrequency handover.

Ø Interfrequency handover.

Ø Intersystem handover.

Ø Hard handover (HHO).

All the old radio links of an MS are released before the new radio links are established.

Soft handover (SHO)

Ø SMS is simultaneously controlled by two or more cells belonging to different BTS of the same RNC or to different RNC.

Ø MS is controlled by at least two cells under one BTS.

Mobile evaluated handover (MEHO)

Ø The UE mainly prepares the handover decision. The final decision is made by SRNC.

Network evaluated handover (NEHO)

Ø The SRNC makes the handover decision.

2.8.3 Why we need RRM or Purpose of RRM?

The rapid increase in the size of the wireless mobile community and its demands for high-speed multimedia communications stands in clear contrast to the rather limited spectrum resources that have been allocated in international agreements. Efficient spectrum or radio resource management (RRM) is of paramount importance due to these increasing demands.

Purposes of RRM are as:

Ø Ensure planned coverage for each service.

Ø Ensure required connection quality.

Ø Ensure planned (low) blocking.

Ø Optimise the system usage in run time.

2.8.4 Joint Radio Resource Management (JRRM)

In designing wireless systems, typical problems are encountered, such as the signal attenuation, terminal noise, fast fading due to the multipath phenomenon, shadowing, Multiple Access Interference (MAI) and other typical system related features, e.g. the mutual relation between interference strength and duration period given by link adaptation. These typical problems challenge the communication systems from using radio resources efficiently. The radio resources not only, by definition, the radio spectrum, but also realized in the real radio networks, access rights for individual mobile users, time period a mobile user being active, channelization codes, transmission power, connection mode, etc., that require the management functions being designed in different time scales. Furthermore, radio resources from different radio networks can be managed jointly in order to solve the encountered problems more effectively. The term Joint Resource Management (JRRM) is therefore generalized as:

JRRM are the controlling mechanisms that support intelligent admission of calls and sessions for a set of networks or cell layers. They control the distribution of traffic, power and the variances of them, thereby aiming at an optimized usage of radio resources and maximized system capacity. JRRM mechanisms work over multiple radio networks or cell layers with the necessary support of reconfigurable/multi-mode terminals. JRRM is operated in a network which consists of several subnetworks or cell layers of a single radio network. The term subnetwork is defined in Definition I-2. The High Performance Radio Local Area Network Type 2 (H/2) as a typical WLAN RAT specified by ETSI (European Telecommunications Standards Institute) BRAN (Broadband Radio Access Networks) and UMTS/FDD specified by the 3rd Generation Partnership Project (3GPP) are two subnetworks studied in this thesis.

2.8.5 Sub-Carrier Allocation

We have analyzed the Dynamic sub-carrier allocation algorithm the main of which is to allocate dynamically a pair of relay and subcarrier to each user-pair in order to minimize the achievable sum rate of each user-pair while satisfying the minimum rate requirement for every user-pair.

2.9 Relay Station and Relay Enhanced Systems

A relay basically is a transceiver which creates a communication link between the source and destination [23]. It virtually can be considered as another transmitter. In third section of this chapter we will be discussing basic idea about Relay, Relay stations, Relaying and its strategies, Co-operative and non-cooperative relaying. But in order to understand these terms one must have idea about Cellular networks, their demand for coverage and capacity and the benefit of using relay station in order to increase the capacity [24].

2.9.1 Cellular Networks

Cellular networks are radio networks made up of a number of non-overlapping cells, each served by at least one base station, that cover a wide geographic area. Several frequencies are assigned to each cell, which can be reused in other cells. Cellular networks mainly consist of two parts: the radio access network or base station subsystem (BSS) and the core network, which are connected through a backhaul connection [24].

2.9.1.1 Analog Transmission to Digital Transmission

Cellular networks appeared in the 1960's and used analog communications. Second generation systems moved from analog to digital due to its many advantages. The components are cheaper, faster, smaller, and require less power. Voice quality is improved due to error correction coding. Digital systems also have higher capacity than analog systems since they can use more spectrally-efficient digital modulation and more efficient techniques to share the cellular spectrum. They can also take advantage of advanced compression techniques and voice activity factors. In addition, encryption techniques can be used to secure digital signals against eavesdropping. Digital systems can also offer data services in addition to voice, including short messaging, e-mail, Internet access, multimedia capabilities, etc [24].

2.9.1.2 Base Station Subsystem

The base station subsystem is responsible for handling traffic and signaling between the core network and the user. It consists of a network of base station transceivers (called nodes) grouped under several base station controllers (BSC or Radio Network Controllers (RNC)) which are connected to the core network. A single BSC can have tens or even hundreds of BTSs under its control. The BSC handles allocation of radio channels, is in charge of admission control, receives measurements from the mobile phones, controls handovers from BTS to BTS, etc. The BSC can route voice calls through the public switched telephone network (PSTN) or provide Internet access. It also acts as a concentrator of low capacity connections to and from the BTS into a high capacity connection to and from the core network. The Base Station is in charge of the radio interface: scrambling, modulation, scheduling, adaptative coding, link quality measurements, soft handovers, etc[24].

2.9.2 Core Network

The core network is in charge of routing and forwarding the user data, handovers between different technologies, it manages the databases with the user and terminals information, security issues, etc. The core network in cellular networks has suffered several changes over the last decade. For example, in the GSM second generation networks a new packet commuting network was added later to give support to GPRS technology which included gateways to external IP networks. Later on, with third generation networks like UMTS and its several releases, the core network was expanded to support soft switching (release 4) or multimedia transmissions (release 5 with the multimedia subsystem). Presently, the core network is evolving to an all-IP network in future generation networks [24].

2.9.4 Coverage Area

The coverage area of a cell is the expected percentage of the cell's area where the received power is greater than a certain minimum, given that the user terminals require a minimum received SNR for acceptable performance.

2.9.5 Capacity

Capacity refers to the theoretical maximum transmission rate that can be achieved over a wireless channel.

2.9.6 Need of using Relay Technology

Presently, the amount of free spectrum is decreasing, and future networks will have to use the available bands at higher frequencies, meaning a decrease in coverage and an increase in base station density. Moreover, next generation cellular networks are expected to support different types of services including web browsing, FTP, video streaming, VoIP, online gaming, real time video, etc., therefore require even higher transmission rates. Physical layer technologies such as OFDM, smart antennas and Multiple Input Multiple Output (MIMO) systems are being designed to achieve this goal. Relay technology can be used both to increase capacity and coverage, as many papers have proved thus far [24].

2.10 Relay

A relay is a node that receives information from the source and forwards it to the destination, so it can assist in a transmission to improve performance. Relays can have many applications, like capacity enhancement, load balancing or coverage extension of cells, which is what we are interested in.

SOURCE RELAY DESTINATION

2.10.1 Why to use Relay

Some reasons which become the reason of deploying relays [25],

Ø Mobile stations can transmit at lower power.

Ø Transmission at higher frequencies is more vulnerable to non LOS conditions.

Ø The transmission power required for high data rates at large distances is very high

Ø Increasing the base station density is one option

Ø A relay acts as a helper node to increase coverage and throughput.

Ø Relay is connected to the base station through wireless channel.

Ø Relays are much closer to the mobile stations than the base station; hence high data rates are possible.

2.10.2 Cooperative relaying

In general, every communicating system has a source node that broadcast the signal towards the multiple/helping relays in the network, which in turn re transmits the processed version towards the destination [26].

2.10.2.1 Benefits of using Cooperative Relaying

Cooperative relaying provides [27],

Ø Better BER performance due to spatial diversity

Ø Higher efficiency due to spatial multiplexing

2.10.2.2 Two methods of Cooperative Relaying

There are two phases through which a signal have to move in order to reach the destination utilizing cooperative environment,

Ø Broadcasting Mode (BA)

When the source broadcasts the signal towards multiple relays present in between destination and source.

Ø Multiple Access Mode (MA)

When different relays transmits their data towards a single destination. This mode of transferring info from multiple relays to single destination is multiple access mode.

2.10.3 Multihop Communication

Multi-hop communication occurs when data travel from the source to the destination node via more than two hops. This could be achieved without need of other costly BSs. The maximum allowed number of hops must be carefully considered (higher number of hops increases a transmission time).

Multi-hop based network may also improve system performance thanks to cooperative relay technique. This is accomplished by sending information simultaneously via multiple different paths and combining the received information at the side of receiver [28].

2.10.4 Relay Station

A relay station is an intermediate station/node that passes information between terminals or other relay stations and is used to help a base station to improve its coverage and capacity. Relay station has other names such as repeater, or multi-hop station and the networks that use relay stations are sometimes called cooperative networks [29].

The notion of relay channel appeared in the late 60's. E. C. van der Meulen introduced the idea of a three-terminal communication channel consisting of a source, a destination and a relay. T. M. Cover and A. El Gamal later published a paper where they computed upper and lower bounds for the capacity of the single relay channel and give an exact expression for the Gaussian degraded case. Given the complexity of the relay channel, it hasn't been until the exhaustive research on MIMO channels over the last decade that relay networks have been on the spot again, and recently there have been many publications on the topic [24].

2.10.4.1 Concept of Relay

Figure 2.3 Relaying System

Ø RS will typically cover a region up to 300 miles in diameter and transmit at lower power level than BS.

Ø Relays do not have wired connection to backhaul.

Ø Cooperative diversity is additional advantage of relaying [25].

2.10.4.2 Relay station vs. Base Station

The primary advantage of deploying relay stations in terms of the cost is expected to come from the differences in the cost of the backhaul. When a relay station is deployed, instead of a Base Station with a wired backhaul connection, there are no direct backhaul costs. There is no cost for provisioning the wired connection, and there are no monthly charges for the backhaul. Similarly, when a relay station is deployed, instead of a Base Station with wireless backhaul, the use of a relay station eliminates the need to purchase, set up, and maintain microwave link equipment, and to purchase the rights to additional spectrum in which this equipment operates. Relay Stations are also expected to be less costly to deploy because they do not require line of sight channel conditions on the relay link, allowing greater flexibility in site selection than for a Base Stations with wireless backhaul.

The idea behind a relay channel is to use the relay to create spatial diversity sending the same signal through independent fading paths. This can be done in two steps. Firstly, the source broadcasts the signal both to the relay and the destination. In the second step, the relay transmits the received signal to the destination, so the destination ends up with two different independent versions of the same signal. The way in which the relay processes and retransmits the received signal to the destination has given rise to several strategies which have been deeply analyzed in the literature [24].

2.10.5 Relaying Strategies

There are many relaying strategies; three of them are discussed below

2.10.5.1 Amplify-and-Forward (AF)

This is the simplest strategy that can be used at the relay because it acts as a dummy with a constraint on the maximum power. The relay amplifies the received signal from the source and transmits it to the destination without doing any decision. The main drawback of this strategy is that the relay terminal is also amplifying the received noise to the destination. When this strategy is applied to the cooperative communication, it is able to obtain a better uncoded bit error rate (BER) than direct transmission [5]. Additionally, the outage probability of the cooperative communication is also derived, demonstrating that a diversity order of two is obtained for two cooperative users. When the relay is equipped with multiple antennas and there is channel state information (CSI) of the source-relay and relay destination links, the AF strategy can attain significant gains over the direct transmission by means of optimum linear filtering the data to be forwarded by the relay [30].

In AF strategy, the relay simply amplifies the noisy version of the signal it has received in the first step and retransmits this noisy version. The destination then combines both signals to decide the transmitted bit. More generally, AF refers to any strategy where the relay linearly transforms the received signal. Although noise is also amplified in the relay, this strategy produces two independent versions of the same signal at the destination which allow for a better detection of the information sent. It has been proved that this strategy is optimal at high SNR, achieving diversity of order two [24].

2.10.5.2 Decode-and-Forward (DF)

In DF strategies, the relay tries to detect the received information, re-encodes and retransmits the signal after detection. If detection is unsuccessful the relaying can be harmful when detecting at the destination, therefore the strategy should be implemented such that the relay only retransmits the information when detection is successful. DF requires more complex devices than AF methods, but the noise at the receiver is much lower [24].

The RS decodes the signal and performs error correction. The decoded data is encoded using the same or different codebook before transmitting to the destination. The two phases need not be of same duration [25].

2.10.5.3 Compress-and-Forward (CF)

In this case, the relay performs a non-linear transformation on the received signal and then retransmits to destination. In this way, the relay station can compress the received signal and forward it to destination without the need to decode it. Some typical examples of CF strategies are “Estimate and Forward” or “Quantize and Forward” [24].

The relay compresses the received signal by using Wyner-Ziv lossy source coding and forwards it to the destination [25].

2.10.5.4 Relaying Techniques (Some Facts)

Ø DF performs well only when the channel quality between BS and the RS is good.

Ø Only advantage of AF is that it is computationally less intensive at the relay.

Ø CF technique performs better than direct transmission in all channel conditions even if channel between BS and RS is degraded.

Ø CF is computationally intensive.

Ø CF is not considered for Wimax relaying [31].

2.10.6 Relay usage Scenarios

These are the Relay usage scenarios created for IEEE 802.16j.

2.10.6.1 Fixed Infrastructure

Fixed-infrastructure relays, like BSs, are to be deployed by the service provider in stationary areas to serve general traffic. They are intended to increase both throughput and coverage because they are likely to be placed above roof tops to allow an LOS with the BS, but this may not always be the case. This category also may include commercial relays purchased by a subscriber, which may leave and enter the network at any time [31].

Fixed Infrastructure is to improve coverage in shadow areas and increase throughput due to LOS communication [25].

2.10.6.2 In-Building Coverage

Even with the relatively small demands of voice service, current mobile phones often perform poorly inside buildings. Relays are expected to be placed both by the service provider and by the end user near the shell, or just inside, of the building to fill the “coverage hole” inside. This type of relay also can be deployed near tunnels or subways to provide coverage where there is otherwise none. These relays can be nomadic and likely will operate with NLOS channels. Intriguingly, they may operate on battery power and probably will have low complexity [31].

In-Building coverage scenario is to fill the coverage hole inside the building [25].

2.10.6.3 Temporary Coverage

Events where a large group of people are densely packed into a small area form a unique opportunity for relays. The multihop capability of 802.16j will enable some of the traffic generated by this dense population to be routed to BSs in adjacent cells. Near stadiums, this infrastructure can be placed by the service provider as a permanent solution. Temporary relays also can be deployed in emergencies where some BSs may have been damaged. For this reason, temporary coverage relays may be required to run on batteries and will range from small and simple to large and complex [31].

Temporary coverage is for the stadiums or gatherings of people during an event. It is also temporary replacement to a damaged relay [25].

2.10.6.4 Coverage on Mobile Vehicle

A mobile vehicle, such as a train or bus, presents unique challenges to communications engineers. Usually, there are several people located very closely together, and the vehicle is moving, sometimes very quickly, through cells. To provide reliable coverage to such users, a complex relay may be deployed on the vehicle and obviously, will be highly mobile [31].

A complex relay is required that can handle quick handoffs while providing coverage in train or bus [25].

2.10.7 Diversity

Multipaths in certain scenarios create fading in the received signal. Due to the high possibility of the multipaths and fading nature of the channel, Diversity technique is being organized which can effectively reduce that fading effects and provides much better reception at the receiver side by achieving same signal trough different channels which in turn with some appropriate combining scheme decreasing probability of BER of the signal and provides better communication result [32].

There are three ways of achieving diversity.

Ø Transmit Diversity

Ø Receive Diversity

Ø MIMO Diversity

2.10.7.1 Cooperative Diversity

Cooperative diversityis a cooperative multiple antenna technique for improving or maximising total networkchannel capacitiesfor any given set of bandwidths which exploits userdiversityby decoding the combined signal of the relayed signal and the direct signal in wireless multihop networks.

2.10.7.2 Spatial Diversity

Spatial diversityis one of several wirelessdiversity schemesthat use two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and indoor environments, there is no clearline-of-sight(LOS) between transmitter and receiver. Instead the signal is reflected along multiple paths before finally being received.

Multiple paths created by use of relay can be used to exploit spatial diversity.

3 OFDM Simulation and Results

In this chapter, we will discuss about simulation model of OFDM transceiver developed in MATLAB and will be discussing the simulation results.

3.1 OFDM Transceiver implementation

Simulation for implementation of basic OFDM transceiver is divided into three portions:

Ø OFDM Transmitter

Ø Channel

Ø OFDM Receiver

3.2 OFDM Transmitter

OFDM Transmitter simulation consists of the following steps as shown is Figure 5.1.

Ø Binary Data Generation

Ø Conversion of Serial input data into Parallel format

Ø Modulation of each symbol using 16 QAM

Ø IFFT block of each modulated symbol

Ø Generation of OFDM signal by combining symbols in serial fashion

3.3 Channel

Channel that we implement in simulation is

§ AWGN with 10 dB SNR

3.4 OFDM Receiver

OFDM Receiver simulation consists of the following steps as shown in Figure 5.2.

§ Reception of Signal through channel and converting into parallel fashion

§ FFT of each symbol

§ QAM demodulation

§ Parallel to Serial Conversion to recover the transmitted data

3.5 Results of Simulation Model

3.5.1 Message Signal

A random Signal is generated using MATLAB built-in function ‘binornd' and is plotted as shown in Figure 5.3

Modulated Signal

Each symbol is passed through the QAM modulator using ‘qammod' built-in function in MATLAB and the scatter plot is obtained as shown in figure 5.4

Signal through Channel

The signal is passed through the channel which is considered as Additive White Gaussian Noise and its scatter plot is given in the Figure 5.5.

Demodulation

The received signal is converted into parallel format and then each symbol is demodulated using MATLAB built-in function ‘qamdemod'. The scatter plot obtained is shown in the Figure 5.6

Recovered Signal

The demodulated symbols are then combined together in order to have the data transmitted data as shown in Figure 5.7

3.6 Conclusion

We have analyzed that how an OFDM signal is generated typically digitally and found that with 10dB SNR signal can be recovered easily but below that SNR there are chances of error in the recovered signal

4 Implementation of Dynamic Subcarrier Allocation ( DSA) Algorithm

In this chapter, we will discuss about the research paper we followed during our final year project and the implementation of the algorithm “Dynamic Subcarrier Allocation (DSA)” proposed in the research paper followed.

4.1 Introduction to Research Paper

During our final year project we have studied many research papers and selected “Subcarrier Allocation for Multiuser Two-Way OFDMA Relay Networks with Fairness Constraints ” by Hanmok Shin and Jae Hong Lee, Seoul National University, Korea, published at VTC.

In this paper we analyzed an adaptive subcarrier allocation scheme for a multiuser two-way OFMDA relay network having multiple user-pairs and multiple relays.

4.1.1 One-way versus Two-way Half duplex relaying systems

4.1.1.1 One-way Half duplex relaying Systems

In one-way half-duplex mode relays don't transmit and receive simultaneously at same time and frequency as shown in Figure 6.1. The main disadvantage of these relaying systems is a loss in throughput compared with full-duplex relaying.

4.1.1.2 Two-way Half duplex relaying Systems

In two-way half-duplex relaying systems two users communicate with each other in two phases:

Ø Phase 1

Phase 1 is the Multiple Access (MA) phase in which all users transmit their information simultaneously to relays.

Ø Phase 2

Phase 2 is the Broadcast (BC) phase in which the relay amplifies the received signal and then broadcast it to all the users using same sub-carrier.

Both these phases are shown in Figure 6.2.

Compared with traditional one-way half-duplex relaying systems, these systems achieve higher power and spectral efficiencies by allowing simultaneous message exchange between a BS and the users [1]. In two-way half-duplex relaying both Amplify and Forward (AF) and Decode and Forward (DF) protocols can be used but AF protocol due to its simple transceiver design is more appealing in practice and is also considered in the paper followed.

4.2 Dynamic Sub-carrier Allocation (DSA ) Algorithm

We have analyzed the Dynamic sub-carrier allocation algorithm the main aim of which is to allocate dynamically a pair of relay and subcarrier to each user-pair in order to maximize the achievable sum-rate of each user-pair while satisfying the minimum rate requirement for every user-pair.

The algorithm is divided into three main steps given below:

Ø Step 1

In the first step, all sets and subcarrier assignment indicator variables are initialized, where the set of user-pairs, relays, and subcarriers are denoted by K , M , and N , respectively.

Set:

K={1,2,….,K}

M = {1, 2, . . .,M}

N={1,2,…,N}

ρk,m(n)= 0

Ø Step 2

In the second step, one relay-subcarrier pair (m*,n*) which maximizes the instantaneous rate (rk,m(n)) is allocated to one user-pair for all user-pairs.

for k = 1 : K

do

(m*,n*) = arg max rk,m(n) , m Є M, n Є N

ρk,m*(n*) = 1 , N = N - {n*};

Update rk

end

where rk is achievable rate of the kth user-pair.

Ø Step 3

In the third step, remaining subcarriers are allocated to the user-pairs and relays under the maximum transmit power constraints of the users and relays.

while N≠∅

do

k*=arg min rk , k Є K

if rk*< rmin

then

(m*,n*) = arg max rk*,m(n) , m Є M, n Є N

ρk*,m*(n*) = 1 , N = N - {n*};

If n=1NpAk*(n)=n=1NpBk*(n)>PU

then

K = K - {k*} ;

end

If n=1NpRm*(n)>PR

then

M = M - {m*};

end

else

n* = rand (N);

(k*,m*) = arg max rk,m n* , m Є M, k Є K

ρk,m*(n*)=1, N = N - { n* };

If n=1NpAk*(n)=n=1NpBk*(n)>PU

then

K = K - {k*} ;

end

If n=1NpRm*(n)>PR

then

M = M - {m*};

end

end

Update rk

end

Where PU and PR denote the maximum transmit power of each of users and relays, respectively. pAk(n) and pBk(n) denote the transmit power of the user Ak and Bk on the subcarrier n, respectively.

4.3 Simulation Framework and Results

4.3.1 Simulation Parameters

We considered the values of parameter in our simulation framework as shown in Table 6.1

Parameter

Value

No. of User-pairs

8

No. of Relays

3

No. of Sub-carriers

128

Minimum rate requirement

2.5 Mbps

Maximum Transmitting power of each user

100 mWatts

Maximum Transmitting power of relay

3000 mWatts

Table 4.1

4.3.2 Simulation Results

We developed the simulation framework to implement the DSA algorithm in MATLAB by employing the above parameters and found the results as shown in Figure 6.3.

Results shows that subcarriers are allocated to each user-pair unless each user-pair or each relay meets its maximum power constraint else all the subcarriers will be allocated to all the user-pairs while satisfying the minimum rate requirement for every user-pair.

5 Conclusions & Future Work

5.1 Conclusion:

Our work basically involved study of relaying and OFDMA technologies. We analyzed adaptive subcarrier allocation algorithm for a multiuser two-way OFDMA relay network. In the DSA algorithm, subcarriers are allocated to the user-pairs with lowest achievable rate to satisfy the minimum rate requirement .We have developed its simulation framework by assigning the subcarriers and relays to the user-pairs depending upon their transmitting powers.

5.2 Future Work:

Working on OFDMA technology to enhance its role in its competitive market is of great importance due to its high data rate feature.