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Radio Resource Management in OFDMA Networks

Disclaimer: This work has been submitted by a student. This is not an example of the work written by our professional academic writers. You can view samples of our professional work here.

Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of UK Essays.

Published: Wed, 21 Feb 2018

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.

  • OFDM is a highly bandwidth efficient scheme because different sub-carriers are orthogonal but they are overlapping.
  • Flexible and can be made adaptive; different modulation schemes for subcarriers, bit loading, adaptable bandwidth/data rates possible.
  • Has excellent ICI performance because of addition of cyclic prefix.
  • In OFDM equalization is performed in frequency domain which becomes very easy as compared to the time domain equalization.
  • Very good at mitigating the effects of delay spread.
  • Due to the use of many sub-carriers, the symbol duration on the sub-carriers is increased, relative to delay spread.
  • ISI is avoided through the use of guard interval.
  • Resistant to frequency selective fading as compared to single carrier system.
  • Used for high data rate transmission.
  • OFDMA provides flexibility of deployment across a variety of frequency bands with little need for modification is of paramount importance.
  • A single frequency network can be used to provide excellent coverage and good frequency re-use.
  • 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],

  • 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.
  • Sensitive to carrier frequency offset, needs frequency offset correction in the receiver.
  • Sensitive to oscillator phase noise, clean and stable oscillator required.
  • The use of guard interval to mitigate ISI affects the bandwidth efficiency.
  • OFDM is sensitive to Doppler shift – frequency errors offset the receiver and if not corrected the orthogonality between the carriers is degraded.
  • If only a few carriers are assigned to each user the resistance to selective fading will be degraded or lost.
  • It has a relatively high sensitivity to frequency offsets as this degrades the orthogonality between the carriers.
  • 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],

  • Not as complicated to implement as OFDM based systems.
  • 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.
  • 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

  • Transmit power
  • Sub carrier allocation
  • Data rates
  • Handover criteria
  • Modulation scheme
  • 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

  • Admission control (AC)
  • Load control (LC)
  • Packet scheduler (PS)
  • 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

  • The interference main resource criteria.
  • LC measures continuously UL and DL interference.
  • RRM acts based on the measurements and parameters from planning

Preventive load control

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

Stable.

Overload condition .

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

Fast LC actions in BTS

  • Lower SIR target for the uplink inner-loop PC.
  • LC actions located in the RNC.
  • Interact with PS and throttle back packet data traffic.
  • Lower bit rates of RT users.(speech service or CS data).
  • WCDMA interfrequency or GSM intersystem handover.
  • Drop single calls in a controlled manner.

2.8.2.3 Connection based functions

  • Handover Control (HC)
  • Power Control (PC)

Power control

  • Uplink open loop power control.
  • Downlink open loop power control.
  • Power in downlink common channels.
  • Uplink inner (closed) loop power control.
  • Downlink inner (closed) loop power control.
  • Outer loop power control.
  • Power control in compressed mode.

Handover

  • Intersystem handover.
  • Intrafrequency handover.
  • Interfrequency handover.
  • Intersystem handover.
  • Hard handover (HHO).

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

Soft handover (SHO)

  • SMS is simultaneously controlled by two or more cells belonging to different BTS of the same RNC or to different RNC.
  • MS is controlled by at least two cells under one BTS.

Mobile evaluated handover (MEHO)

  • The UE mai

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